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 bf18c10b569463f33abadb53a221eb3b150b8ca8..7dbc572ec123728d5b3e082b40e5d7d46696f9a4 100644
--- a/sdk/lib/_internal/compiler/implementation/util/util.dart
+++ b/sdk/lib/_internal/compiler/implementation/util/util.dart
@@ -194,3 +194,20 @@ int longestCommonPrefixLength(List a, List b) {
   }
   return index;
 }
+
+/// 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;
+}