dart2js: add --preserve-uris flag.

sdk/lib/_internal/compiler/implementation/js_backend/backend.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
index 6e567f577e8ab88ad3273653d62ba0e8ff837b90..0375ea8d3696bbaee7a331f3aff4a6a0d207e6a4 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
@@ -362,8 +362,9 @@ class JavaScriptBackend extends Backend {
   /// program, this variable will stil be false.
   bool hasRetainedMetadata = false;
 
-  /// True if a call to preserveUris has been seen.
-  bool mustRetainUris = false;
+  /// True if a call to preserveUris has been seen and the preserve-uris flag
+  /// is set.
+  bool mustPreserveUris = false;
 
   /// True if a call to preserveLibraryNames has been seen.
   bool mustRetainLibraryNames = false;
@@ -1568,7 +1569,7 @@ class JavaScriptBackend extends Backend {
     } else if (element == preserveMetadataMarker) {
       mustRetainMetadata = true;
     } else if (element == preserveUrisMarker) {
-      mustRetainUris = true;
+      if (compiler.preserveUris) mustPreserveUris = true;
     } else if (element == preserveLibraryNamesMarker) {
       mustRetainLibraryNames = true;
     } else if (element == getIsolateAffinityTagMarker) {