Format throw and rethrow. (#1347)

* Format throw and rethrow.

* Space() instead.

* Added another throw test.

Author: kallentu

lib/src/front_end/ast_node_visitor.dart

@@ -1497,7 +1497,7 @@ class AstNodeVisitor extends ThrowingAstVisitor<Piece> with PieceFactory {
 
   @override
   Piece visitRethrowExpression(RethrowExpression node) {
-    throw UnimplementedError();
+    return tokenPiece(node.rethrowKeyword);
   }
 
   @override
@@ -1712,7 +1712,11 @@ class AstNodeVisitor extends ThrowingAstVisitor<Piece> with PieceFactory {
 
   @override
   Piece visitThrowExpression(ThrowExpression node) {
-    throw UnimplementedError();
+    return buildPiece((b) {
+      b.token(node.throwKeyword);
+      b.space();
+      b.visit(node.expression);
+    });
   }
 
   @override

test/statement/other.stmt

@@ -55,3 +55,29 @@ foo() async {
 foo() async {
   await i(1 + 2);
 }
+>>> Throw.
+throw   'error'
+;
+<<<
+throw 'error';
+>>> Throw doesn't split after the 'throw' keyword.
+throw 'Some extremely long error message.';
+<<<
+throw 'Some extremely long error message.';
+>>> Throw with long string literal.
+throw new FormatException('This is a long exception message.');
+<<<
+throw new FormatException(
+  'This is a long exception message.',
+);
+>>> Rethrow.
+try {
+  throw 1   ;
+} catch    (e) {
+       rethrow    ;}
+<<<
+try {
+  throw 1;
+} catch (e) {
+  rethrow;
+}