Emit to StreamCodeOutput instead of CodeBuffer.

pkg/compiler/lib/src/deferred_load.dart

Index: pkg/compiler/lib/src/deferred_load.dart
diff --git a/pkg/compiler/lib/src/deferred_load.dart b/pkg/compiler/lib/src/deferred_load.dart
index f11b6545e07036c08ab9558dca600908930a6545..6ae1f75311667edc18a3a921073b67e589d91700 100644
--- a/pkg/compiler/lib/src/deferred_load.dart
+++ b/pkg/compiler/lib/src/deferred_load.dart
@@ -74,10 +74,15 @@ class OutputUnit {
   /// loaded.
   final Setlet<Import> imports = new Setlet<Import>();
 
+  /// `true` if this output unit is for the main output file.
+  final bool isMainOutput;
+
   /// A unique name representing this [OutputUnit].
   /// Based on the set of [imports].
   String name;
 
+  OutputUnit({this.isMainOutput: false});
+
   String toString() => "OutputUnit($name)";
 
   bool operator==(OutputUnit other) {
@@ -110,7 +115,7 @@ class DeferredLoadTask extends CompilerTask {
       new LiteralDartString("main")), null, null, null);
 
   /// The OutputUnit that will be loaded when the program starts.
-  final OutputUnit mainOutputUnit = new OutputUnit();
+  final OutputUnit mainOutputUnit = new OutputUnit(isMainOutput: true);
 
   /// A set containing (eventually) all output units that will result from the
   /// program.