Update comments in core lib showing proper factory syntax.

runtime/lib/error.dart

Index: runtime/lib/error.dart
===================================================================
--- runtime/lib/error.dart	(revision 1677)
+++ runtime/lib/error.dart	(working copy)
@@ -12,8 +12,8 @@
   static _throwNew(int assertionStart, int assertionEnd)
       native "AssertionError_throwNew";
   String toString() {
-    return "Failed assertion: '$failedAssertion' is not true " +
-        "in $url at line $line, column $column.";
+    return "'$url': Failed assertion: line $line pos $column: " +
+        "'$failedAssertion' is not true.";
   }
   final String failedAssertion;
   final String url;
@@ -27,9 +27,8 @@
         "TypeError can only be allocated by the VM");
   }
   String toString() {
-    return "Failed type check: type $srcType is not assignable to type " +
-        "$dstType of $dstName in $url at line " +
-        "$line, column $column.";
+    return "'$url': Failed type check: line $line pos $column: " +
+        "type '$srcType' is not assignable to type '$dstType' of '$dstName'.";
   }
   final String srcType;
   final String dstType;
@@ -43,7 +42,7 @@
   }
   static _throwNew(int case_clause_pos) native "FallThroughError_throwNew";
   String toString() {
-    return "Switch case fall-through in $url at line $line.";
+    return "'$url': Switch case fall-through at line $line.";
   }
   final String url;
   final int line;