Format yield and yield*. (#1312)

* Format yield and yield*.

* Fix yield tests and then yield itself.

Turns out the one liner yield statement doesn't parse as a yield and
instead parses it as a variable declaration (with yield being the type).

Change tests to be properly parsing yield statements and then fixed up
the behaviour for it.

Author: kallentu

lib/src/front_end/ast_node_visitor.dart

@@ -1335,7 +1335,11 @@ class AstNodeVisitor extends ThrowingAstVisitor<void>
 
   @override
   void visitYieldStatement(YieldStatement node) {
-    throw UnimplementedError();
+    token(node.yieldKeyword);
+    token(node.star);
+    space();
+    visit(node.expression);
+    token(node.semicolon);
   }
 
   /// If [node] is not `null`, then visit it.

test/statement/other.stmt

@@ -30,4 +30,20 @@ while (true) {
 <<<
 while (true) {
   continue someLabel;
-}
+}
+>>> Yield.
+Stream<String> i(String n) async* {
+  yield   i   ;
+}
+<<<
+Stream<String> i(String n) async* {
+  yield i;
+}
+>>> Yield*.
+Stream<int> i(int n) async* {
+  yield * i (  n   - 1 )      ;
+}
+<<<
+Stream<int> i(int n) async* {
+  yield* i(n - 1);
+}

test/statement/other_comment.stmt

@@ -35,3 +35,19 @@ while (true) {
 while (true) {
   break; // comment
 }
+>>> Yield with comment after semicolon.
+Stream<String> i(String n) async* {
+  yield   i   ;     // comment
+}
+<<<
+Stream<String> i(String n) async* {
+  yield i; // comment
+}
+>>> Yield* with comment after semicolon.
+Stream<int> i(int n) async* {
+  yield  *   i( x )    ;    // comment
+}
+<<<
+Stream<int> i(int n) async* {
+  yield* i(x); // comment
+}