Format native methods. (#1340)

Author: munificent

lib/src/front_end/ast_node_visitor.dart

@@ -1162,7 +1162,11 @@ class AstNodeVisitor extends ThrowingAstVisitor<Piece> with PieceFactory {
 
   @override
   Piece visitNativeFunctionBody(NativeFunctionBody node) {
-    throw UnimplementedError();
+    return buildPiece((b) {
+      b.token(node.nativeKeyword);
+      b.visit(node.stringLiteral, spaceBefore: true);
+      b.token(node.semicolon);
+    });
   }
 
   @override

test/declaration/native.unit

@@ -0,0 +1,25 @@
+40 columns                              |
+>>> Without strings.
+class Foo {
+  Foo()   native   ;
+  int bar()   native ;
+}
+<<<
+class Foo {
+  Foo() native;
+  int bar() native;
+}
+>>> With strings.
+class Foo {
+  Foo()   native  "Foo" ;
+  int bar()   native  "bar"   ;
+}
+<<<
+class Foo {
+  Foo() native "Foo";
+  int bar() native "bar";
+}
+>>> class
+class Foo    native   "Foo"  {  }
+<<<
+class Foo native "Foo" {}