When emitting an arrow function, parenthesize the body if it could be interpreted as a block instead of an object literal.

src/compiler/emitter.ts

@@ -4202,9 +4202,19 @@ module ts {
                     return;
                 }
 
-                // For es6 and higher we can emit the expression as is.
+                // For es6 and higher we can emit the expression as is.  However, in the case 
+                // where the expression might end up looking like a block when down-leveled, we'll
+                // also wrap it in parentheses first.  For example if you have: a => <foo>{}
+                // then we need to generate: a => ({})
                 write(" ");
-                emit(body);
+
+                // Unwrap all type assertions.
+                var current = body;
+                while (current.kind === SyntaxKind.TypeAssertionExpression) {
+                    current = (<TypeAssertion>current).expression;
+                }
+
+                emitParenthesizedIf(body, current.kind === SyntaxKind.ObjectLiteralExpression);
             }
 
             function emitDownLevelExpressionFunctionBody(node: FunctionLikeDeclaration, body: Expression) {