Allow dart2dart to output one file per library.

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

Index: sdk/lib/_internal/compiler/implementation/util/util.dart
diff --git a/sdk/lib/_internal/compiler/implementation/util/util.dart b/sdk/lib/_internal/compiler/implementation/util/util.dart
index 04854e6ea602afeb0138cbc1c2953da572353158..c766739799edf7b73195d572be3d0d1e660b6774 100644
--- a/sdk/lib/_internal/compiler/implementation/util/util.dart
+++ b/sdk/lib/_internal/compiler/implementation/util/util.dart
@@ -165,3 +165,19 @@ String modifiersToString({bool isStatic: false,
   builder.toLink().printOn(buffer, ', ');
   return buffer.toString();
 }
+
+// Returns suggestedName if it is not in usedNames. Otherwise concatenates
+// the smallest number that makes it not appear in usedNames.
+// Adds the result to usedNames.
+String makeUnique(String suggestedName, Set<String> usedNames) {
+  String result = suggestedName;
+  if (usedNames.contains(suggestedName)) {
+    int counter = 0;
+    while (usedNames.contains(result)) {
+      counter++;
+      result = "$suggestedName$counter";
+    }
+  }
+  usedNames.add(result);
+  return result;
+}