diff --git a/lib/src/io_utils.dart b/lib/src/io_utils.dart
index b0d02225a6..207fb3dc8a 100644
--- a/lib/src/io_utils.dart
+++ b/lib/src/io_utils.dart
@@ -5,6 +5,8 @@
 /// This is a helper library to make working with io easier.
 library dartdoc.io_utils;
 
+import 'dart:async';
+import 'dart:convert';
 import 'dart:io';
 
 import 'package:path/path.dart' as path;
@@ -60,3 +62,158 @@ String getFileNameFor(String name) =>
 final libraryNameRegexp = new RegExp('[.:]');
 final partOfRegexp = new RegExp('part of ');
 final newLinePartOfRegexp = new RegExp('\npart of ');
+
+final RegExp quotables = new RegExp(r'[ "\r\n\$]');
+
+class SubprocessLauncher {
+  final String context;
+  Map<String, String> _environment;
+
+  Map<String, String> get environment => _environment;
+
+  String get prefix => context.isNotEmpty ? '$context: ' : '';
+
+  // from flutter:dev/tools/dartdoc.dart, modified
+  static void _printStream(Stream<List<int>> stream, Stdout output,
+      {String prefix: '', Iterable<String> Function(String line) filter}) {
+    assert(prefix != null);
+    if (filter == null) filter = (line) => [line];
+    stream
+        .transform(UTF8.decoder)
+        .transform(const LineSplitter())
+        .expand(filter)
+        .listen((String line) {
+      if (line != null) {
+        output.write('$prefix$line'.trim());
+        output.write('\n');
+      }
+    });
+  }
+
+  SubprocessLauncher(this.context, [Map<String, String> environment]) {
+    if (environment == null) this._environment = new Map();
+  }
+
+  /// A wrapper around start/await process.exitCode that will display the
+  /// output of the executable continuously and fail on non-zero exit codes.
+  /// It will also parse any valid JSON objects (one per line) it encounters
+  /// on stdout/stderr, and return them.  Returns null if no JSON objects
+  /// were encountered.
+  ///
+  /// Makes running programs in grinder similar to set -ex for bash, even on
+  /// Windows (though some of the bashisms will no longer make sense).
+  /// TODO(jcollins-g): move this to grinder?
+  Future<Iterable<Map>> runStreamed(String executable, List<String> arguments,
+      {String workingDirectory}) async {
+    List<Map> jsonObjects;
+
+    /// Allow us to pretend we didn't pass the JSON flag in to dartdoc by
+    /// printing what dartdoc would have printed without it, yet storing
+    /// json objects into [jsonObjects].
+    Iterable<String> jsonCallback(String line) {
+      Map result;
+      try {
+        result = json.decoder.convert(line);
+      } catch (FormatException) {}
+      if (result != null) {
+        if (jsonObjects == null) {
+          jsonObjects = new List();
+        }
+        jsonObjects.add(result);
+        if (result.containsKey('message')) {
+          line = result['message'];
+        } else if (result.containsKey('data')) {
+          line = result['data']['text'];
+        }
+      }
+      return line.split('\n');
+    }
+
+    stderr.write('$prefix+ ');
+    if (workingDirectory != null) stderr.write('(cd "$workingDirectory" && ');
+    if (environment != null) {
+      stderr.write(environment.keys.map((String key) {
+        if (environment[key].contains(quotables)) {
+          return "$key='${environment[key]}'";
+        } else {
+          return "$key=${environment[key]}";
+        }
+      }).join(' '));
+      stderr.write(' ');
+    }
+    stderr.write('$executable');
+    if (arguments.isNotEmpty) {
+      for (String arg in arguments) {
+        if (arg.contains(quotables)) {
+          stderr.write(" '$arg'");
+        } else {
+          stderr.write(" $arg");
+        }
+      }
+    }
+    if (workingDirectory != null) stderr.write(')');
+    stderr.write('\n');
+    Process process = await Process.start(executable, arguments,
+        workingDirectory: workingDirectory, environment: environment);
+
+    _printStream(process.stdout, stdout, prefix: prefix, filter: jsonCallback);
+    _printStream(process.stderr, stderr, prefix: prefix, filter: jsonCallback);
+    await process.exitCode;
+
+    int exitCode = await process.exitCode;
+    if (exitCode != 0) {
+      throw new ProcessException(executable, arguments,
+          "SubprocessLauncher got non-zero exitCode", exitCode);
+    }
+    return jsonObjects;
+  }
+}
+
+/// Output formatter for comparing warnings.
+String printWarningDelta(String title, String dartdocOriginalBranch,
+    Map<String, int> original, Map<String, int> current) {
+  StringBuffer printBuffer = new StringBuffer();
+  Set<String> quantityChangedOuts = new Set();
+  Set<String> onlyOriginal = new Set();
+  Set<String> onlyCurrent = new Set();
+  Set<String> allKeys =
+      new Set.from([]..addAll(original.keys)..addAll(current.keys));
+
+  for (String key in allKeys) {
+    if (original.containsKey(key) && !current.containsKey(key)) {
+      onlyOriginal.add(key);
+    } else if (!original.containsKey(key) && current.containsKey(key)) {
+      onlyCurrent.add(key);
+    } else if (original.containsKey(key) &&
+        current.containsKey(key) &&
+        original[key] != current[key]) {
+      quantityChangedOuts.add(key);
+    }
+  }
+
+  if (onlyOriginal.isNotEmpty) {
+    printBuffer.writeln(
+        '*** $title : ${onlyOriginal.length} warnings from original ($dartdocOriginalBranch) missing in current:');
+    onlyOriginal.forEach((warning) => printBuffer.writeln(warning));
+  }
+  if (onlyCurrent.isNotEmpty) {
+    printBuffer.writeln(
+        '*** $title : ${onlyCurrent.length} new warnings not in original ($dartdocOriginalBranch)');
+    onlyCurrent.forEach((warning) => printBuffer.writeln(warning));
+  }
+  if (quantityChangedOuts.isNotEmpty) {
+    printBuffer.writeln('*** $title : Identical warning quantity changed');
+    for (String key in quantityChangedOuts) {
+      printBuffer.writeln(
+          "* Appeared ${original[key]} times in original ($dartdocOriginalBranch), now ${current[key]}:");
+      printBuffer.writeln(key);
+    }
+  }
+  if (onlyOriginal.isEmpty &&
+      onlyCurrent.isEmpty &&
+      quantityChangedOuts.isEmpty) {
+    printBuffer.writeln(
+        '*** $title : No difference in warning output from original ($dartdocOriginalBranch)${allKeys.isEmpty ? "" : " (${allKeys.length} warnings found)"}');
+  }
+  return printBuffer.toString();
+}
diff --git a/test/io_utils_test.dart b/test/io_utils_test.dart
index 5b0548c4ca..a550e249fe 100644
--- a/test/io_utils_test.dart
+++ b/test/io_utils_test.dart
@@ -17,4 +17,36 @@ void main() {
       expect(getFileNameFor('dartdoc.generator'), 'dartdoc-generator.html');
     });
   });
+
+  group('printWarningDelta', () {
+    Map<String, int> original;
+    Map<String, int> current;
+    setUp(() {
+      original = new Map.fromIterables(
+          ["originalwarning", "morewarning", "duplicateoriginalwarning"],
+          [1, 1, 2]);
+      current = new Map.fromIterables(
+          ["newwarning", "morewarning", "duplicateoriginalwarning"], [1, 1, 1]);
+    });
+
+    test('verify output of printWarningDelta', () {
+      expect(
+          printWarningDelta('Diff Title', 'oldbranch', original, current),
+          equals(
+              '*** Diff Title : 1 warnings from original (oldbranch) missing in current:\n'
+              'originalwarning\n'
+              '*** Diff Title : 1 new warnings not in original (oldbranch)\n'
+              'newwarning\n'
+              '*** Diff Title : Identical warning quantity changed\n'
+              '* Appeared 2 times in original (oldbranch), now 1:\n'
+              'duplicateoriginalwarning\n'));
+    });
+
+    test('verify output when nothing changes', () {
+      expect(
+          printWarningDelta('Diff Title 2', 'oldbranch2', original, original),
+          equals(
+              '*** Diff Title 2 : No difference in warning output from original (oldbranch2) (3 warnings found)\n'));
+    });
+  });
 }
diff --git a/tool/grind.dart b/tool/grind.dart
index d84e372b72..f7278379d6 100644
--- a/tool/grind.dart
+++ b/tool/grind.dart
@@ -3,7 +3,6 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
-import 'dart:convert';
 import 'dart:io' hide ProcessException;
 
 import 'package:dartdoc/src/io_utils.dart';
@@ -25,23 +24,19 @@ Directory get dartdocDocsDir =>
 Directory get sdkDocsDir => tempdirsCache.memoized1(createTempSync, 'sdkdocs');
 Directory get flutterDir => tempdirsCache.memoized1(createTempSync, 'flutter');
 
+/// Version of dartdoc we should use when making comparisons.
+String get dartdocOriginalBranch {
+  String branch = 'master';
+  if (Platform.environment.containsKey('DARTDOC_ORIGINAL')) {
+    branch = Platform.environment['DARTDOC_ORIGINAL'];
+    log('using branch/tag: $branch for comparison from \$DARTDOC_ORIGINAL');
+  }
+  return branch;
+}
+
 final Directory flutterDirDevTools =
     new Directory(path.join(flutterDir.path, 'dev', 'tools'));
 
-final RegExp quotables = new RegExp(r'[ "\r\n\$]');
-// from flutter:dev/tools/dartdoc.dart, modified
-void _printStream(Stream<List<int>> stream, Stdout output,
-    {String prefix: ''}) {
-  assert(prefix != null);
-  stream
-      .transform(UTF8.decoder)
-      .transform(const LineSplitter())
-      .listen((String line) {
-    output.write('$prefix$line'.trim());
-    output.write('\n');
-  });
-}
-
 /// Creates a throwaway pub cache and returns the environment variables
 /// necessary to use it.
 Map<String, String> _createThrowawayPubCache() {
@@ -57,66 +52,9 @@ Map<String, String> _createThrowawayPubCache() {
   ]);
 }
 
-class _SubprocessLauncher {
-  final String context;
-  Map<String, String> _environment;
-
-  Map<String, String> get environment => _environment;
-
-  String get prefix => context.isNotEmpty ? '$context: ' : '';
-
-  _SubprocessLauncher(this.context, [Map<String, String> environment]) {
-    if (environment == null) this._environment = new Map();
-  }
-
-  /// A wrapper around start/await process.exitCode that will display the
-  /// output of the executable continuously and fail on non-zero exit codes.
-  /// Makes running programs in grinder similar to set -ex for bash, even on
-  /// Windows (though some of the bashisms will no longer make sense).
-  /// TODO(jcollins-g): move this to grinder?
-  Future runStreamed(String executable, List<String> arguments,
-      {String workingDirectory}) async {
-    stderr.write('$prefix+ ');
-    if (workingDirectory != null) stderr.write('cd "$workingDirectory" && ');
-    if (environment != null) {
-      stderr.write(environment.keys.map((String key) {
-        if (environment[key].contains(quotables)) {
-          return "$key='${environment[key]}'";
-        } else {
-          return "$key=${environment[key]}";
-        }
-      }).join(' '));
-      stderr.write(' ');
-    }
-    stderr.write('$executable');
-    if (arguments.isNotEmpty) {
-      for (String arg in arguments) {
-        if (arg.contains(quotables)) {
-          stderr.write(" '$arg'");
-        } else {
-          stderr.write(" $arg");
-        }
-      }
-    }
-    if (workingDirectory != null) stderr.write(')');
-    stderr.write('\n');
-    Process process = await Process.start(executable, arguments,
-        workingDirectory: workingDirectory, environment: environment);
-
-    _printStream(process.stdout, stdout, prefix: prefix);
-    _printStream(process.stderr, stderr, prefix: prefix);
-    await process.exitCode;
-
-    int exitCode = await process.exitCode;
-    if (exitCode != 0) {
-      fail("exitCode: $exitCode");
-    }
-  }
-}
-
 @Task('Analyze dartdoc to ensure there are no errors and warnings')
 analyze() async {
-  await new _SubprocessLauncher('analyze').runStreamed(
+  await new SubprocessLauncher('analyze').runStreamed(
     sdkBin('dartanalyzer'),
     [
       '--fatal-warnings',
@@ -135,22 +73,84 @@ buildbot() => null;
 @Task('Generate docs for the Dart SDK')
 Future buildSdkDocs() async {
   log('building SDK docs');
-  var launcher = new _SubprocessLauncher('build-sdk-docs');
-  await launcher.runStreamed(Platform.resolvedExecutable, [
-    '--checked',
-    'bin/dartdoc.dart',
-    '--output',
-    '${sdkDocsDir.path}',
-    '--sdk-docs',
-    '--show-progress'
-  ]);
+  await _buildSdkDocs(
+      sdkDocsDir.path, new Future.value(Directory.current.path));
+}
+
+/// Returns a map of warning texts to the number of times each has been seen.
+Map<String, int> jsonMessageIterableToWarnings(Iterable<Map> messageIterable) {
+  Map<String, int> warningTexts = new Map();
+  for (Map<String, dynamic> message in messageIterable) {
+    if (message.containsKey('level') &&
+        message['level'] == 'WARNING' &&
+        message.containsKey('data')) {
+      warningTexts.putIfAbsent(message['data']['text'], () => 0);
+      warningTexts[message['data']['text']]++;
+    }
+  }
+  return warningTexts;
+}
+
+@Task('Display delta in SDK warnings')
+Future compareSdkWarnings() async {
+  Directory originalDartdocSdkDocs =
+      Directory.systemTemp.createTempSync('dartdoc-comparison-sdkdocs');
+  Future originalDartdoc = createComparisonDartdoc();
+  Future currentDartdocSdkBuild = _buildSdkDocs(
+      sdkDocsDir.path, new Future.value(Directory.current.path), 'current');
+  Future originalDartdocSdkBuild =
+      _buildSdkDocs(originalDartdocSdkDocs.path, originalDartdoc, 'original');
+  Map<String, int> currentDartdocWarnings =
+      jsonMessageIterableToWarnings(await currentDartdocSdkBuild);
+  Map<String, int> originalDartdocWarnings =
+      jsonMessageIterableToWarnings(await originalDartdocSdkBuild);
+
+  print(printWarningDelta('SDK docs', dartdocOriginalBranch,
+      originalDartdocWarnings, currentDartdocWarnings));
+}
+
+/// Helper function to create a clean version of dartdoc (based on the current
+/// directory, assumed to be a git repository).  Uses [dartdocOriginalBranch]
+/// to checkout a branch or tag.
+Future<String> createComparisonDartdoc() async {
+  var launcher = new SubprocessLauncher('create-comparison-dartdoc');
+  Directory dartdocClean =
+      Directory.systemTemp.createTempSync('dartdoc-comparison');
+  await launcher
+      .runStreamed('git', ['clone', Directory.current.path, dartdocClean.path]);
+  await launcher.runStreamed('git', ['checkout', dartdocOriginalBranch],
+      workingDirectory: dartdocClean.path);
+  await launcher.runStreamed(sdkBin('pub'), ['get'],
+      workingDirectory: dartdocClean.path);
+  return dartdocClean.path;
+}
+
+Future<List<Map>> _buildSdkDocs(String sdkDocsPath, Future<String> futureCwd,
+    [String label]) async {
+  if (label == null) label = '';
+  if (label != '') label = '-$label';
+  var launcher = new SubprocessLauncher('build-sdk-docs$label');
+  String cwd = await futureCwd;
+  await launcher.runStreamed(sdkBin('pub'), ['get'], workingDirectory: cwd);
+  return await launcher.runStreamed(
+      Platform.resolvedExecutable,
+      [
+        '--checked',
+        'bin/dartdoc.dart',
+        '--output',
+        '${sdkDocsDir.path}',
+        '--sdk-docs',
+        '--json',
+        '--show-progress',
+      ],
+      workingDirectory: cwd);
 }
 
 @Task('Serve generated SDK docs locally with dhttpd on port 8000')
 @Depends(buildSdkDocs)
 Future serveSdkDocs() async {
   log('launching dhttpd on port 8000 for SDK');
-  var launcher = new _SubprocessLauncher('serve-sdk-docs');
+  var launcher = new SubprocessLauncher('serve-sdk-docs');
   await launcher.runStreamed(sdkBin('pub'), [
     'run',
     'dhttpd',
@@ -165,7 +165,7 @@ Future serveSdkDocs() async {
 @Depends(buildFlutterDocs)
 Future serveFlutterDocs() async {
   log('launching dhttpd on port 8001 for Flutter');
-  var launcher = new _SubprocessLauncher('serve-flutter-docs');
+  var launcher = new SubprocessLauncher('serve-flutter-docs');
   await launcher.runStreamed(sdkBin('pub'), ['get']);
   await launcher.runStreamed(sdkBin('pub'), [
     'run',
@@ -180,42 +180,47 @@ Future serveFlutterDocs() async {
 @Task('Build flutter docs')
 Future buildFlutterDocs() async {
   log('building flutter docs into: $flutterDir');
-  var launcher =
-      new _SubprocessLauncher('build-flutter-docs', _createThrowawayPubCache());
+  await _buildFlutterDocs(flutterDir.path);
+  String index =
+      new File(path.join(flutterDir.path, 'dev', 'docs', 'doc', 'index.html'))
+          .readAsStringSync();
+  stdout.write(index);
+}
+
+Future _buildFlutterDocs(String flutterPath, [String label]) async {
+  var launcher = new SubprocessLauncher(
+      'build-flutter-docs${label == null ? "" : "-$label"}',
+      _createThrowawayPubCache());
   await launcher.runStreamed('git',
       ['clone', '--depth', '1', 'https://github.com/flutter/flutter.git', '.'],
-      workingDirectory: flutterDir.path);
+      workingDirectory: flutterPath);
   String flutterBin = path.join('bin', 'flutter');
   String flutterCacheDart =
-      path.join(flutterDir.path, 'bin', 'cache', 'dart-sdk', 'bin', 'dart');
+      path.join(flutterPath, 'bin', 'cache', 'dart-sdk', 'bin', 'dart');
   String flutterCachePub =
-      path.join(flutterDir.path, 'bin', 'cache', 'dart-sdk', 'bin', 'pub');
+      path.join(flutterPath, 'bin', 'cache', 'dart-sdk', 'bin', 'pub');
   await launcher.runStreamed(
     flutterBin,
     ['--version'],
-    workingDirectory: flutterDir.path,
+    workingDirectory: flutterPath,
   );
   await launcher.runStreamed(
     flutterBin,
     ['precache'],
-    workingDirectory: flutterDir.path,
+    workingDirectory: flutterPath,
   );
   await launcher.runStreamed(
     flutterCachePub,
     ['get'],
-    workingDirectory: path.join(flutterDir.path, 'dev', 'tools'),
+    workingDirectory: path.join(flutterPath, 'dev', 'tools'),
   );
   await launcher
       .runStreamed(flutterCachePub, ['global', 'activate', '-spath', '.']);
   await launcher.runStreamed(
     flutterCacheDart,
     [path.join('dev', 'tools', 'dartdoc.dart')],
-    workingDirectory: flutterDir.path,
+    workingDirectory: flutterPath,
   );
-  String index =
-      new File(path.join(flutterDir.path, 'dev', 'docs', 'doc', 'index.html'))
-          .readAsStringSync();
-  stdout.write(index);
 }
 
 @Task('Checks that CHANGELOG mentions current version')
@@ -326,13 +331,13 @@ publish() async {
 test() async {
   // `pub run test` is a bit slower than running an `test_all.dart` script
   // But it provides more useful output in the case of failures.
-  await new _SubprocessLauncher('test')
+  await new SubprocessLauncher('test')
       .runStreamed(sdkBin('pub'), ['run', 'test']);
 }
 
 @Task('Generate docs for dartdoc')
 testDartdoc() async {
-  var launcher = new _SubprocessLauncher('test-dartdoc');
+  var launcher = new SubprocessLauncher('test-dartdoc');
   await launcher.runStreamed(Platform.resolvedExecutable,
       ['--checked', 'bin/dartdoc.dart', '--output', dartdocDocsDir.path]);
   File indexHtml = joinFile(dartdocDocsDir, ['index.html']);
@@ -341,7 +346,7 @@ testDartdoc() async {
 
 @Task('update test_package_docs')
 updateTestPackageDocs() async {
-  var launcher = new _SubprocessLauncher('update-test-package-docs');
+  var launcher = new SubprocessLauncher('update-test-package-docs');
   var testPackageDocs =
       new Directory(path.join('testing', 'test_package_docs'));
   var testPackage = new Directory(path.join('testing', 'test_package'));