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

src/main/java/com/google/devtools/build/lib/syntax/Parser.java

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,

src/test/java/com/google/devtools/build/lib/syntax/EvaluationTest.java

diff --git a/src/test/java/com/google/devtools/build/lib/syntax/EvaluationTest.java b/src/test/java/com/google/devtools/build/lib/syntax/EvaluationTest.java
index 496e30b..04746bb 100644
--- a/src/test/java/com/google/devtools/build/lib/syntax/EvaluationTest.java
+++ b/src/test/java/com/google/devtools/build/lib/syntax/EvaluationTest.java
@@ -430,6 +430,11 @@
   }
 
   @Test
+  public void testSingleTuple() throws Exception {
+    newTest().setUp("a, = [1]").testLookup("a", 1);
+  }
+
+  @Test
   public void testHeterogeneousDict() throws Exception {
     newTest().setUp("d = {'str': 1, 2: 3}", "a = d['str']", "b = d[2]").testLookup("a", 1)
         .testLookup("b", 3);