Delete dart_backend from compiler.

tests/compiler/dart2js/mock_compiler.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library mock_compiler;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:compiler/compiler.dart' as api;
@@ skipping 62 matching lines @@
   final Measurer measurer = new Measurer();
 
   MockCompiler.internal(
       {Map<String, String> coreSource,
        bool enableTypeAssertions: false,
        bool enableUserAssertions: false,
        bool enableMinification: false,
        bool disableTypeInference: false,
        bool analyzeAll: false,
        bool analyzeOnly: false,
-       bool emitJavaScript: true,
        bool preserveComments: false,
        // Our unit tests check code generation output that is
        // affected by inlining support.
        bool disableInlining: true,
        bool trustTypeAnnotations: false,
        bool trustJSInteropTypeAnnotations: false,
        bool enableAsyncAwait: false,
        int this.expectedWarnings,
        int this.expectedErrors,
        api.CompilerOutputProvider outputProvider,
        String patchVersion,
        LibrarySourceProvider this.librariesOverride})
       : sourceFiles = new Map<String, SourceFile>(),
         testedPatchVersion = patchVersion,
         super(options: new CompilerOptions(
               entryPoint: new Uri(scheme: 'mock'),
               libraryRoot: Uri.parse('placeholder_library_root_for_mock/'),
               enableTypeAssertions: enableTypeAssertions,
               enableUserAssertions: enableUserAssertions,
               disableInlining: disableInlining,
               enableAssertMessage: true,
               enableMinification: enableMinification,
               disableTypeInference: disableTypeInference,
               analyzeAll: analyzeAll,
               analyzeOnly: analyzeOnly,
-              emitJavaScript: emitJavaScript,
               preserveComments: preserveComments,
               trustTypeAnnotations: trustTypeAnnotations,
               trustJSInteropTypeAnnotations: trustJSInteropTypeAnnotations,
               shownPackageWarnings: const []),
               outputProvider: new LegacyCompilerOutput(outputProvider)) {
 
     deferredLoadTask = new MockDeferredLoadTask(this);
 
     registerSource(Uris.dart_core,
                    buildLibrarySource(DEFAULT_CORE_LIBRARY, coreSource));
@@ skipping 284 matching lines @@
       trustTypeAnnotations: trustTypeAnnotations,
       enableTypeAssertions: enableTypeAssertions,
       enableUserAssertions: enableUserAssertions,
       expectedErrors: expectedErrors,
       expectedWarnings: expectedWarnings,
       outputProvider: outputProvider);
   compiler.registerSource(uri, code);
   compiler.diagnosticHandler = createHandler(compiler, code);
   return compiler;
 }