Add --hide-package-warnings option.

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 2da85e1cdd82e289694ea10898eaf9e7aca0255a..b44541509d61898caba2c840451aa51e13d98d03 100644
--- a/sdk/lib/_internal/compiler/implementation/dart2js.dart
+++ b/sdk/lib/_internal/compiler/implementation/dart2js.dart
@@ -279,6 +279,7 @@ Future compile(List<String> argv) {
     new OptionHandler('--dump-info', passThrough),
     new OptionHandler('--disallow-unsafe-eval',
                       (_) => hasDisallowUnsafeEval = true),
+    new OptionHandler('--hide-package-warnings', passThrough),
     new OptionHandler('-D.+=.*', addInEnvironment),
 
     // The following two options must come last.
@@ -557,6 +558,9 @@ Supported options:
     Emit diagnostics without suggestions for how to get rid of the diagnosed
     problems.
 
+  --hide-package-warnings
+    Hide warnings and hints generated from packages.
+
 The following options are only used for compiler development and may
 be removed in a future version: