dart2js: add --preserve-uris flag.

sdk/lib/_internal/compiler/implementation/dart2js.dart

Index: sdk/lib/_internal/compiler/implementation/dart2js.dart
diff --git a/sdk/lib/_internal/compiler/implementation/dart2js.dart b/sdk/lib/_internal/compiler/implementation/dart2js.dart
index ef6e8faa2bdf2ea306ca085636c3248ffbce8a1f..99c6754ae0ce5f3b3fb8aa9bc42cf6afb26f15b7 100644
--- a/sdk/lib/_internal/compiler/implementation/dart2js.dart
+++ b/sdk/lib/_internal/compiler/implementation/dart2js.dart
@@ -288,6 +288,7 @@ Future compile(List<String> argv) {
     new OptionHandler('--out=.+|-o.*', setOutput, multipleArguments: true),
     new OptionHandler('--allow-mock-compilation', passThrough),
     new OptionHandler('--minify|-m', implyCompilation),
+    new OptionHandler('--preserve-uris', passThrough),
     new OptionHandler('--force-strip=.*', setStrip),
     new OptionHandler('--disable-diagnostic-colors',
                       (_) => diagnosticHandler.enableColors = false),
@@ -524,6 +525,10 @@ Supported options:
   --show-package-warnings
     Show warnings and hints generated from packages.
 
+  --preserve-uris
+    Preserve the source URIs in the reflection data. Without this flag the
+    `uri` getter for `LibraryMirror`s is mangled in minified mode.
+
   --csp
     Disables dynamic generation of code in the generated output. This is
     necessary to satisfy CSP restrictions (see http://www.w3.org/TR/CSP/).