Setup correct scope for initializers and complain about fields initialized more than once.

pkg/front_end/lib/src/fasta/kernel/kernel_formal_parameter_builder.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_formal_parameter_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_formal_parameter_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_formal_parameter_builder.dart
index fc014eb7748add4aaa5d9ba3a62a75c4159ed2bb..2bd7426315038c1e95c3da23f3e71a0811cef196 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_formal_parameter_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_formal_parameter_builder.dart
@@ -7,6 +7,8 @@ library fasta.kernel_formal_parameter_builder;
 import 'package:front_end/src/fasta/kernel/kernel_shadow_ast.dart'
     show KernelVariableDeclaration;
 
+import '../modifier.dart' show finalMask;
+
 import 'kernel_builder.dart'
     show
         FormalParameterBuilder,
@@ -47,4 +49,14 @@ class KernelFormalParameterBuilder
     }
     return declaration;
   }
+
+  @override
+  FormalParameterBuilder forFormalParameterInitializerScope() {
+    assert(declaration != null);
+    return !hasThis
+        ? this
+        : (new KernelFormalParameterBuilder(metadata, modifiers | finalMask,
+            type, name, hasThis, parent, charOffset)
+          ..declaration = declaration);
+  }
 }