Some refactoring of how import specifiers are parsed as per code review feedback

Author: sheetalkamat

Diff for: src/compiler/parser.ts

@@ -4656,11 +4656,21 @@ module ts {
             // ImportSpecifier:
             //  ImportedBinding
             //  IdentifierName as ImportedBinding
-            if (lookAhead(nextTokenIsAsKeyword)) {
-                node.propertyName = parseIdentifierName();
+            var isfirstIdentifierNameNotAnIdentifier = isKeyword(token) && !isIdentifier();
+            var start = scanner.getTokenPos();
+            var identifierName = parseIdentifierName();
+            if (token === SyntaxKind.AsKeyword) {
+                node.propertyName = identifierName;
                 parseExpected(SyntaxKind.AsKeyword);
+                node.name = parseIdentifier();
+            }
+            else {
+                node.name = identifierName;
+                if (isfirstIdentifierNameNotAnIdentifier) {
+                    // Report error identifier expected
+                    parseErrorAtPosition(start, identifierName.end - start, Diagnostics.Identifier_expected);
+                }
             }
-            node.name = parseIdentifier();
             return finishNode(node);
         }
 

Diff for: tests/baselines/reference/es6ImportNamedImportIdentifiersParsing.errors.txt

@@ -0,0 +1,18 @@
+tests/cases/compiler/es6ImportNamedImportIdentifiersParsing.ts(3,10): error TS1003: Identifier expected.
+tests/cases/compiler/es6ImportNamedImportIdentifiersParsing.ts(4,19): error TS1003: Identifier expected.
+tests/cases/compiler/es6ImportNamedImportIdentifiersParsing.ts(6,21): error TS1003: Identifier expected.
+
+
+==== tests/cases/compiler/es6ImportNamedImportIdentifiersParsing.ts (3 errors) ====
+    
+    import { yield } from "somemodule"; // Allowed
+    import { default } from "somemodule"; // Error - as this is keyword that is not allowed as identifier
+             ~~~~~~~
+!!! error TS1003: Identifier expected.
+    import { yield as default } from "somemodule"; // error to use default as binding name
+                      ~~~~~~~
+!!! error TS1003: Identifier expected.
+    import { default as yield } from "somemodule"; // no error 
+    import { default as default } from "somemodule"; // default as is ok, error of default binding name
+                        ~~~~~~~
+!!! error TS1003: Identifier expected.

Diff for: tests/cases/compiler/es6ImportNamedImportIdentifiersParsing.ts

@@ -0,0 +1,8 @@
+// @target: es6
+// @module: commonjs
+
+import { yield } from "somemodule"; // Allowed
+import { default } from "somemodule"; // Error - as this is keyword that is not allowed as identifier
+import { yield as default } from "somemodule"; // error to use default as binding name
+import { default as yield } from "somemodule"; // no error 
+import { default as default } from "somemodule"; // default as is ok, error of default binding name