analyzer: Report unnecessary import directives.

In a given library, every import directive has a set of "used
elements," the subset of elements provided by the import which are
used in the library. In a given library, an import directive is
"unnecessary" if there exists at least one other import directive
with the same prefix as the aforementioned import directive, and a
"used elements" set which is a proper superset of the
aforementioned import directive's "used elements" set.

#44569

Change-Id: I4da43be36837cdafb07a0ab67a94e644f351e593
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/177221
Commit-Queue: Samuel Rawlins <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: srawlins

pkg/analysis_server/test/src/services/correction/fix/add_missing_required_argument_test.dart

@@ -33,7 +33,6 @@ class AddMissingRequiredArgumentTest extends FixProcessorTest {
   Future<void> test_constructor_flutter_children() async {
     await resolveTestCode('''
 import 'package:flutter/widgets.dart';
-import 'package:meta/meta.dart';
 
 class MyWidget extends Widget {
   MyWidget({@required List<Widget> children});
@@ -45,7 +44,6 @@ build() {
 ''');
     await assertHasFix('''
 import 'package:flutter/widgets.dart';
-import 'package:meta/meta.dart';
 
 class MyWidget extends Widget {
   MyWidget({@required List<Widget> children});
@@ -60,7 +58,6 @@ build() {
   Future<void> test_constructor_flutter_hasTrailingComma() async {
     await resolveTestCode('''
 import 'package:flutter/widgets.dart';
-import 'package:meta/meta.dart';
 
 class MyWidget extends Widget {
   MyWidget({@required int a, @required int b});
@@ -72,7 +69,6 @@ build() {
 ''');
     await assertHasFix('''
 import 'package:flutter/widgets.dart';
-import 'package:meta/meta.dart';
 
 class MyWidget extends Widget {
   MyWidget({@required int a, @required int b});
@@ -335,7 +331,6 @@ main() {
   Future<void> test_param_child() async {
     await resolveTestCode('''
 import 'package:flutter/widgets.dart';
-import 'package:meta/meta.dart';
 
 class MyWidget extends Widget {
   MyWidget({@required String foo, @required Widget child});
@@ -349,7 +344,6 @@ build() {
 ''');
     await assertHasFix('''
 import 'package:flutter/widgets.dart';
-import 'package:meta/meta.dart';
 
 class MyWidget extends Widget {
   MyWidget({@required String foo, @required Widget child});
@@ -367,7 +361,6 @@ build() {
   Future<void> test_param_children() async {
     await resolveTestCode('''
 import 'package:flutter/widgets.dart';
-import 'package:meta/meta.dart';
 
 class MyWidget extends Widget {
   MyWidget({@required String foo, @required List<Widget> children});
@@ -381,7 +374,6 @@ build() {
 ''');
     await assertHasFix('''
 import 'package:flutter/widgets.dart';
-import 'package:meta/meta.dart';
 
 class MyWidget extends Widget {
   MyWidget({@required String foo, @required List<Widget> children});

pkg/analyzer/lib/error/error.dart

@@ -592,6 +592,7 @@ const List<ErrorCode> errorCodeValues = [
   HintCode.UNIGNORABLE_IGNORE,
   HintCode.UNNECESSARY_CAST,
   HintCode.UNNECESSARY_IGNORE,
+  HintCode.UNNECESSARY_IMPORT,
   HintCode.UNNECESSARY_NO_SUCH_METHOD,
   HintCode.UNNECESSARY_NULL_COMPARISON_FALSE,
   HintCode.UNNECESSARY_NULL_COMPARISON_TRUE,

pkg/analyzer/lib/src/dart/analysis/library_analyzer.dart

@@ -299,6 +299,8 @@ class LibraryAnalyzer {
       verifier.generateDuplicateShownHiddenNameHints(errorReporter);
       verifier.generateUnusedImportHints(errorReporter);
       verifier.generateUnusedShownNameHints(errorReporter);
+      verifier.generateUnnecessaryImportHints(
+          errorReporter, _usedImportedElementsList);
     }
 
     // Unused local elements.

pkg/analyzer/lib/src/dart/error/hint_codes.dart

@@ -2684,6 +2684,12 @@ class HintCode extends AnalyzerErrorCode {
           "Try removing the name from the list, or removing the whole comment "
           "if this is the only name in the list.");
 
+  static const HintCode UNNECESSARY_IMPORT = HintCode(
+      'UNNECESSARY_IMPORT',
+      "The import of '{0}' is unnecessary as all of the used elements are also "
+          "provided by the import of '{1}'.",
+      correction: 'Try removing the import directive.');
+
   /**
    * Unnecessary `noSuchMethod` declaration.
    */