Only check source file when we have one.

Author: CyrusNajmabadi

src/compiler/core.ts

@@ -301,8 +301,11 @@ module ts {
 
         Debug.assert(start >= 0, "start must be non-negative, is " + start);
         Debug.assert(length >= 0, "length must be non-negative, is " + length);
-        Debug.assert(start <= file.text.length, `start must be within the bounds of the file. ${ start } > ${ file.text.length }`);
-        Debug.assert(end <= file.text.length, `end must be the bounds of the file. ${ end } > ${ file.text.length }`);
+
+        if (file) {
+            Debug.assert(start <= file.text.length, `start must be within the bounds of the file. ${ start } > ${ file.text.length }`);
+            Debug.assert(end <= file.text.length, `end must be the bounds of the file. ${ end } > ${ file.text.length }`);
+        }
 
         let text = getLocaleSpecificMessage(message.key);
 

src/compiler/utilities.ts

@@ -165,6 +165,10 @@ module ts {
             return "";
         }
 
+        if (!sourceFile && node.kind === SyntaxKind.Identifier) {
+            return (<Identifier>node).text;
+        }
+
         let text = sourceFile.text;
         return text.substring(skipTrivia(text, node.pos), node.end);
     }