Allow dart2dart to output one file per library.

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

Index: sdk/lib/_internal/compiler/implementation/apiimpl.dart
diff --git a/sdk/lib/_internal/compiler/implementation/apiimpl.dart b/sdk/lib/_internal/compiler/implementation/apiimpl.dart
index 6643920966f93af0e798e9cbb01be62b52f157b0..ad3e10c9991b4e0cdc055b60df77604d24993aac 100644
--- a/sdk/lib/_internal/compiler/implementation/apiimpl.dart
+++ b/sdk/lib/_internal/compiler/implementation/apiimpl.dart
@@ -45,7 +45,9 @@ class Compiler extends leg.Compiler {
             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'),