Add support for FieldFormalParameter.

pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart

Index: pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart b/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
index fcf2b398008b902142cea0f4bd52e7c48b7fd21d..e808728f7f174a1e4f55d1f1e42b01bbb59d62dc 100644
--- a/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
+++ b/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
@@ -491,17 +491,32 @@ class AstBuilder extends ScopeListener {
     push(ast.throwExpression(toAnalyzerToken(throwToken), pop()));
   }
 
-  void endFormalParameter(Token thisKeyword) {
+  void endFormalParameter(Token thisKeyword, Token period) {
     debugEvent("FormalParameter");
-    if (thisKeyword != null) {
-      internalError("'this' can't be used here.");
-    }
     SimpleIdentifier name = pop();
     TypeName type = pop();
     Token keyword = _popOptionalSingleModifier();
     pop(); // Metadata.
-    SimpleFormalParameter node = ast.simpleFormalParameter(
-        null, null, toAnalyzerToken(keyword), type, name);
+
+    FormalParameter node;
+    if (thisKeyword == null) {
+      node = ast.simpleFormalParameter(
+          null, null, toAnalyzerToken(keyword), type, name);
+    } else {
+      TypeParameterList typeParameters; // TODO(scheglov)
+      FormalParameterList formalParameters; // TODO(scheglov)
+      node = ast.fieldFormalParameter(
+          null,
+          null,
+          toAnalyzerToken(keyword),
+          type,
+          toAnalyzerToken(thisKeyword),
+          toAnalyzerToken(period),
+          name,
+          typeParameters,
+          formalParameters);
+    }
+
     scope[name.name] = name.staticElement = new AnalyzerParameterElement(node);
     push(node);
   }