Prevent await expression from being treated as an assignment target, fixes #240 (#241)

Author: Protryon

src/main/java/com/shapesecurity/shift/es2017/parser/GenericParser.java

@@ -1442,6 +1442,7 @@ protected Either3<Expression, Pair<FormalParameters, Boolean>, AssignmentTarget>
         AdditionalStateT startState = this.startNode();
 
         if (this.allowAwaitExpression && this.eat(TokenType.AWAIT)) {
+            this.isBindingElement = this.isAssignmentTarget = false;
             Either3<Expression, Pair<FormalParameters, Boolean>, AssignmentTarget> expression = this.isolateCoverGrammar(this::parseUnaryExpression);
             return Either3.left(this.finishNode(startState, new AwaitExpression(expression.left().fromJust())));
         }

src/test/java/com/shapesecurity/shift/es2017/parser/miscellaneous/AsyncAwaitTest.java

@@ -102,6 +102,7 @@ public void testAsyncMisc() throws JsError {
 		testScript("async ((a))", new CallExpression(new IdentifierExpression("async"), ImmutableList.of(new IdentifierExpression("a"))));
 		testScript("async function a(){}(0)", new Script(ImmutableList.empty(), ImmutableList.of(new FunctionDeclaration(true, false, new BindingIdentifier("a"),  new FormalParameters(ImmutableList.empty(), Maybe.empty()), new FunctionBody(ImmutableList.empty(), ImmutableList.empty())), new ExpressionStatement(new LiteralNumericExpression(0.0)))));
 		testScript("(async function a(){}(0))", new CallExpression(new FunctionExpression(true, false, Maybe.of(new BindingIdentifier("a")),  new FormalParameters(ImmutableList.empty(), Maybe.empty()), new FunctionBody(ImmutableList.empty(), ImmutableList.empty())), ImmutableList.of(new LiteralNumericExpression(0.0))));
+		testScript("(async function() { (await y); })", new FunctionExpression(true, false, Maybe.empty(), new FormalParameters(ImmutableList.empty(), Maybe.empty()), new FunctionBody(ImmutableList.empty(), ImmutableList.of(new ExpressionStatement(new AwaitExpression(new IdentifierExpression("y")))))));
 	}
 
 	@Test