Make mappedBy lazy.

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

Index: sdk/lib/_internal/compiler/implementation/ssa/codegen.dart
diff --git a/sdk/lib/_internal/compiler/implementation/ssa/codegen.dart b/sdk/lib/_internal/compiler/implementation/ssa/codegen.dart
index 3cc1318ff01e66a3bd61a42a691e24bd00231c8c..cec2d7329c5c2dc5decc12f9585d14f1aeac28de 100644
--- a/sdk/lib/_internal/compiler/implementation/ssa/codegen.dart
+++ b/sdk/lib/_internal/compiler/implementation/ssa/codegen.dart
@@ -1137,7 +1137,7 @@ abstract class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
    * Sequentialize a list of conceptually parallel copies. Parallel
    * copies may contain cycles, that this method breaks.
    */
-  void sequentializeCopies(List<Copy> copies,
+  void sequentializeCopies(Iterable<Copy> copies,
                            String tempName,
                            void doAssignment(String target, String source)) {
     // Map to keep track of the current location (ie the variable that
@@ -1220,7 +1220,7 @@ abstract class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
     if (handler == null) return;
 
     // Map the instructions to strings.
-    List<Copy> copies = handler.copies.mappedBy((Copy copy) {
+    Iterable<Copy> copies = handler.copies.mappedBy((Copy copy) {
       return new Copy(variableNames.getName(copy.source),
                       variableNames.getName(copy.destination));
     });