Make an empty identifier a scanning error.

lib/compiler/implementation/scanner/scanner.dart

Index: lib/compiler/implementation/scanner/scanner.dart
diff --git a/lib/compiler/implementation/scanner/scanner.dart b/lib/compiler/implementation/scanner/scanner.dart
index 84af8ad0ab1f9ca936711c4a9a960fd85aa11810..7b04e873b220a3236d2f72df27a9a5a40a88502e 100644
--- a/lib/compiler/implementation/scanner/scanner.dart
+++ b/lib/compiler/implementation/scanner/scanner.dart
@@ -653,6 +653,11 @@ class AbstractScanner<T extends SourceString> implements Scanner {
           (next === $$ && allowDollar)) {
         next = advance();
       } else if (next < 128) {
+        // Identifier ends here.
+        if (start == byteOffset) {
+          throw new MalformedInputException("expected identifier not found",
+                                            charOffset);
+        }
         if (isAscii) {
           appendByteStringToken(IDENTIFIER_INFO, asciiString(start, 0));
         } else {