Allow construct "a, = [1]" in parser.

This is equivalent to:
  (a,) = [1]
which puts the element of a 1-element list in the variable.

--
MOS_MIGRATED_REVID=103371728
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/Parser.java b/src/main/java/com/google/devtools/build/lib/syntax/Parser.java
index d685dcf..078efce 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/Parser.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/Parser.java
@@ -101,6 +101,7 @@
       EnumSet.of(
           TokenKind.EOF,
           TokenKind.NEWLINE,
+          TokenKind.EQUALS,
           TokenKind.RBRACE,
           TokenKind.RBRACKET,
           TokenKind.RPAREN,