Remove deprecated IllegalJSRegExpException class.

sdk/lib/_internal/compiler/implementation/lib/regexp_helper.dart

Index: sdk/lib/_internal/compiler/implementation/lib/regexp_helper.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/regexp_helper.dart b/sdk/lib/_internal/compiler/implementation/lib/regexp_helper.dart
index b62b0b5c5b46e2d45906f891556561f8d88101ae..1435fe425b60ae34e14c9333fdbabf52c950980b 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/regexp_helper.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/regexp_helper.dart
@@ -40,8 +40,9 @@ regExpMakeNative(JSSyntaxRegExp regExp, {bool global: false}) {
   try {
     return JS('var', r'new RegExp(#, #)', pattern, sb.toString());
   } catch (e) {
-    throw new IllegalJSRegExpException(pattern,
-                                       JS('String', r'String(#)', e));
+    String errorMessage = JS('String', r'String(#)', e);
+    throw new FormatException(
+        "Illegal RegExp pattern: $pattern, $errorMessage");
   }
 }