Allow dart2dart to output one file per library.

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

Index: sdk/lib/_internal/compiler/implementation/compiler.dart
diff --git a/sdk/lib/_internal/compiler/implementation/compiler.dart b/sdk/lib/_internal/compiler/implementation/compiler.dart
index e7e74bdc3c2bc9c36b66d7d516725ac008777106..830b075da9f0364a11b1659d37820fe1482c30bc 100644
--- a/sdk/lib/_internal/compiler/implementation/compiler.dart
+++ b/sdk/lib/_internal/compiler/implementation/compiler.dart
@@ -916,6 +916,7 @@ abstract class Compiler implements DiagnosticListener {
             this.enableMinification: false,
             this.enableNativeLiveTypeAnalysis: false,
             bool emitJavaScript: true,
+            bool dart2dartMultiFile: false,
             bool generateSourceMap: true,
             bool analyzeAllFlag: false,
             bool analyzeOnly: false,
@@ -962,7 +963,8 @@ abstract class Compiler implements DiagnosticListener {
       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.