Add fatal-warnings flag, that turns warnings into compilation errors (the name

pkg/compiler/lib/src/dart2js.dart

Index: pkg/compiler/lib/src/dart2js.dart
diff --git a/pkg/compiler/lib/src/dart2js.dart b/pkg/compiler/lib/src/dart2js.dart
index 6f3d3fa2f744a1ee56f55859da5c463361fbb01e..2c46faab1e1d297184ea9e57e04c028229f0b191 100644
--- a/pkg/compiler/lib/src/dart2js.dart
+++ b/pkg/compiler/lib/src/dart2js.dart
@@ -296,6 +296,7 @@ Future<api.CompilationResult> compile(List<String> argv) {
       diagnosticHandler.showWarnings = false;
       passThrough('--suppress-warnings');
     }),
+    new OptionHandler('--fatal-warnings', passThrough),
     new OptionHandler('--suppress-hints',
                       (_) => diagnosticHandler.showHints = false),
     new OptionHandler(
@@ -562,6 +563,9 @@ Supported options:
   --suppress-warnings
     Do not display any warnings.
 
+  --fatal-warnings
+    Treat warnings as compilation errors.
+
   --suppress-hints
     Do not display any hints.