Cache DirectoryBasedDartSdk sources.

This reduces analysis time from of a trivial Dart app from 822 ms
to 762 ms. With SDK summaries.

[email protected]
BUG=

Review URL: https://codereview.chromium.org/1577363002 .

Author: scheglov

pkg/analyzer/lib/src/generated/sdk_io.dart

@@ -4,6 +4,7 @@
 
 library analyzer.src.generated.sdk_io;
 
+import 'dart:collection';
 import 'dart:io';
 
 import 'package:analyzer/src/context/context.dart';
@@ -200,6 +201,11 @@ class DirectoryBasedDartSdk implements DartSdk {
    */
   JavaFile _sdkDirectory;
 
+  /**
+   * The directory within the SDK directory that contains the libraries.
+   */
+  JavaFile _libraryDirectory;
+
   /**
    * The revision number of this SDK, or `"0"` if the revision number cannot be
    * discovered.
@@ -231,6 +237,11 @@ class DirectoryBasedDartSdk implements DartSdk {
    */
   LibraryMap _libraryMap;
 
+  /**
+   * The mapping from Dart URI's to the corresponding sources.
+   */
+  Map<String, Source> _uriToSourceMap = new HashMap<String, Source>();
+
   /**
    * Initialize a newly created SDK to represent the Dart SDK installed in the
    * [sdkDirectory]. The flag [useDart2jsPaths] is `true` if the dart2js path
@@ -333,8 +344,13 @@ class DirectoryBasedDartSdk implements DartSdk {
   /**
    * Return the directory within the SDK directory that contains the libraries.
    */
-  JavaFile get libraryDirectory =>
-      new JavaFile.relative(_sdkDirectory, _LIB_DIRECTORY_NAME);
+  JavaFile get libraryDirectory {
+    if (_libraryDirectory == null) {
+      _libraryDirectory =
+          new JavaFile.relative(_sdkDirectory, _LIB_DIRECTORY_NAME);
+    }
+    return _libraryDirectory;
+  }
 
   /**
    * Return the file containing the Pub executable, or `null` if it does not exist.
@@ -514,6 +530,35 @@ class DirectoryBasedDartSdk implements DartSdk {
 
   @override
   Source mapDartUri(String dartUri) {
+    Source source = _uriToSourceMap[dartUri];
+    if (source == null) {
+      source = _mapDartUri(dartUri);
+      _uriToSourceMap[dartUri] = source;
+    }
+    return source;
+  }
+
+  /**
+   * Return the [SdkBundle] for this SDK, if it exists, or `null` otherwise.
+   */
+  SdkBundle _getSummarySdkBundle() {
+    String rootPath = directory.getAbsolutePath();
+    String path = pathos.join(rootPath, 'lib', '_internal', 'analysis_summary');
+    try {
+      File file = new File(path);
+      if (file.existsSync()) {
+        List<int> bytes = file.readAsBytesSync();
+        return new SdkBundle.fromBuffer(bytes);
+      }
+    } catch (exception, stackTrace) {
+      AnalysisEngine.instance.logger.logError(
+          'Failed to load SDK analysis summary from $path',
+          new CaughtException(exception, stackTrace));
+    }
+    return null;
+  }
+
+  FileBasedSource _mapDartUri(String dartUri) {
     String libraryName;
     String relativePath;
     int index = dartUri.indexOf('/');
@@ -540,26 +585,6 @@ class DirectoryBasedDartSdk implements DartSdk {
     }
   }
 
-  /**
-   * Return the [SdkBundle] for this SDK, if it exists, or `null` otherwise.
-   */
-  SdkBundle _getSummarySdkBundle() {
-    String rootPath = directory.getAbsolutePath();
-    String path = pathos.join(rootPath, 'lib', '_internal', 'analysis_summary');
-    try {
-      File file = new File(path);
-      if (file.existsSync()) {
-        List<int> bytes = file.readAsBytesSync();
-        return new SdkBundle.fromBuffer(bytes);
-      }
-    } catch (exception, stackTrace) {
-      AnalysisEngine.instance.logger.logError(
-          'Failed to load SDK analysis summary from $path',
-          new CaughtException(exception, stackTrace));
-    }
-    return null;
-  }
-
   /**
    * Return the given [file] if it exists and is executable, or `null` if it
    * does not exist or is not executable.