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

pkg/compiler/lib/src/compiler.dart

Index: pkg/compiler/lib/src/compiler.dart
diff --git a/pkg/compiler/lib/src/compiler.dart b/pkg/compiler/lib/src/compiler.dart
index 6bbcd29a28ca97fef1c43155b35014a77cc2a64a..eef0520b38009d1f280718df23ca7795bacadf5c 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -749,6 +749,7 @@ abstract class Compiler implements DiagnosticListener {
   Map<Uri, SuppressionInfo> suppressedWarnings = <Uri, SuppressionInfo>{};
 
   final bool suppressWarnings;
+  final bool fatalWarnings;
 
   /// If `true`, some values are cached for reuse in incremental compilation.
   /// Incremental compilation is basically calling [run] more than once.
@@ -1009,6 +1010,7 @@ abstract class Compiler implements DiagnosticListener {
             this.showPackageWarnings: false,
             this.useContentSecurityPolicy: false,
             this.suppressWarnings: false,
+            this.fatalWarnings: false,
             bool hasIncrementalSupport: false,
             this.enableExperimentalMirrors: false,
             this.allowNativeExtensions: false,