Allow dart2dart to output one file per library.

sdk/lib/_internal/compiler/implementation/compiler.dart

 // Copyright (c) 2012, 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.
 
 part of dart2js;
 
 /**
  * If true, print a warning for each method that was resolved, but not
  * compiled.
  */
@@ skipping 898 matching lines @@
 
   Compiler({this.enableTypeAssertions: false,
             this.enableUserAssertions: false,
             this.trustTypeAnnotations: false,
             this.enableConcreteTypeInference: false,
             bool disableTypeInferenceFlag: false,
             this.maxConcreteTypeSize: 5,
             this.enableMinification: false,
             this.enableNativeLiveTypeAnalysis: false,
             bool emitJavaScript: true,
+            bool dart2dartMultiFile: false,
             bool generateSourceMap: true,
             bool analyzeAllFlag: false,
             bool analyzeOnly: false,
             this.analyzeMain: false,
             bool analyzeSignaturesOnly: false,
             this.preserveComments: false,
             this.verbose: false,
             this.sourceMapUri: null,
             this.outputUri: null,
             this.buildId: UNDETERMINED_BUILD_ID,
@@ skipping 26 matching lines @@
         new CodegenRegistry(this, new TreeElementMapping(null));
 
     closureMapping.ClosureNamer closureNamer;
     if (emitJavaScript) {
       js_backend.JavaScriptBackend jsBackend =
           new js_backend.JavaScriptBackend(this, generateSourceMap);
       closureNamer = jsBackend.namer;
       backend = jsBackend;
     } else {
       closureNamer = new closureMapping.ClosureNamer();
-      backend = new dart_backend.DartBackend(this, strips);
+      backend = new dart_backend.DartBackend(this, strips,
+                                             multiFile: dart2dartMultiFile);
     }
 
     // No-op in production mode.
     validator = new TreeValidatorTask(this);
 
     tasks = [
       libraryLoader = new LibraryLoaderTask(this),
       scanner = new ScannerTask(this),
       dietParser = new DietParserTask(this),
       parser = new ParserTask(this),
@@ skipping 1126 matching lines @@
   static NullSink outputProvider(String name, String extension) {
     return new NullSink('$name.$extension');
   }
 }
 
 /// Information about suppressed warnings and hints for a given library.
 class SuppressionInfo {
   int warnings = 0;
   int hints = 0;
 }