Allow dart2dart to output one file per library.

sdk/lib/_internal/compiler/implementation/apiimpl.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.
 
 library leg_apiimpl;
 
 import 'dart:async';
 
 import '../compiler.dart' as api;
 import 'dart2jslib.dart' as leg;
@@ skipping 24 matching lines @@
         this.allowedLibraryCategories = getAllowedLibraryCategories(options),
         super(
             outputProvider: outputProvider,
             enableTypeAssertions: hasOption(options, '--enable-checked-mode'),
             enableUserAssertions: hasOption(options, '--enable-checked-mode'),
             trustTypeAnnotations:
                 hasOption(options, '--trust-type-annotations'),
             enableMinification: hasOption(options, '--minify'),
             enableNativeLiveTypeAnalysis:
                 !hasOption(options, '--disable-native-live-type-analysis'),
-            emitJavaScript: !hasOption(options, '--output-type=dart'),
+            emitJavaScript: !(hasOption(options, '--output-type=dart') ||
+                              hasOption(options, '--output-type=dart-multi')),
+            dart2dartMultiFile: hasOption(options, '--output-type=dart-multi'),
             generateSourceMap: !hasOption(options, '--no-source-maps'),
             analyzeAllFlag: hasOption(options, '--analyze-all'),
             analyzeOnly: hasOption(options, '--analyze-only'),
             analyzeMain: hasOption(options, '--analyze-main'),
             analyzeSignaturesOnly:
                 hasOption(options, '--analyze-signatures-only'),
             strips: extractCsvOption(options, '--force-strip='),
             enableConcreteTypeInference:
                 hasOption(options, '--enable-concrete-type-inference'),
             disableTypeInferenceFlag:
@@ skipping 274 matching lines @@
   }
 
   void diagnoseCrashInUserCode(String message, exception, stackTrace) {
     hasCrashed = true;
     print('$message: ${tryToString(exception)}');
     print(tryToString(stackTrace));
   }
 
   fromEnvironment(String name) => environment[name];
 }