Fixes issues with multiple await return.

src/NUglify.Tests/JavaScript/ES2017.cs

@@ -35,5 +35,11 @@ public void AsyncArrowFunction()
         {
             TestHelper.Instance.RunTest();
         }
+
+        [Test]
+        public void MultipleAwaitExpression()
+        {
+            TestHelper.Instance.RunTest();
+        }
     }
 }

src/NUglify.Tests/NUglify.Tests.csproj

@@ -687,6 +687,9 @@
     <Content Include="TestData\JS\Expected\ES2017\AsyncFunctionExpressions.js">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </Content>
+    <Content Include="TestData\JS\Expected\ES2017\MultipleAwaitExpression.js">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </Content>
     <Content Include="TestData\JS\Expected\ES2017\TrailingComma.js">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </Content>
@@ -3014,6 +3017,9 @@
     <Content Include="TestData\JS\Input\ES2017\AsyncArrowFunction.js">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </Content>
+    <Content Include="TestData\JS\Input\ES2017\MultipleAwaitExpression.js">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </Content>
     <Content Include="TestData\JS\Input\ES2017\AsyncClassFunction.js">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </Content>

src/NUglify.Tests/TestData/JS/Input/ES2017/MultipleAwaitExpression.js

@@ -0,0 +1,16 @@
+async function getNumberAfterTimeout(value) {
+    return new Promise((resolve) => {
+        window.setTimeout(() => resolve(value), 2000);
+    });
+}
+
+async function sum(x) {
+    var a = getNumberAfterTimeout(20);
+    var b = getNumberAfterTimeout(30);
+    return x + await a + await b;
+}
+
+(async () => { return x + await a(t) + await b; })()
+
+// keep grouping
+(async () => { return x + (await a(t) + await b); })()

src/NUglify/JavaScript/JSParser.cs

@@ -4344,30 +4344,21 @@ private AstNode ParseAwaitExpression()
         {
             ParsedVersion = ScriptVersion.EcmaScript6;
 
-            // save the context of the yield operator, then skip past it
+            // save the context of the await operator, then skip past it
             var context = m_currentToken.Clone();
             var operatorContext = context.Clone();
             GetNextToken();
 
-            // must be followed by an expression
-            var expression = ParseExpression(true);
-            if (expression == null)
-            {
-                // we only call this method if we KNOW we are ES6, so if there is no expression,
-                // then throw an error.
-                ReportError(JSError.ExpressionExpected);
-            }
-            else
-            {
-                context.UpdateWith(expression.Context);
-            }
+            bool bAssign;
+            AstNode expression = ParseUnaryExpression(out bAssign, false);
+            expression = new UnaryExpression(expression.Context.CombineWith(operatorContext))
+                {
+                    OperatorContext = operatorContext,
+                    OperatorToken = JSToken.Await,
+                    Operand = expression
+                };
 
-            return new UnaryExpression(context)
-            {
-                OperatorContext = operatorContext,
-                OperatorToken = JSToken.Await,
-                Operand = expression
-            };
+            return expression;
         }
 
         private FunctionObject ParseArrowFunction(AstNode parameters)