Enable support for multiple variable assignments.

This happens in for loops, list comprehensions and simple assigment.
e.g.
  a, b = expr
  [a for a, b in expr]

--
MOS_MIGRATED_REVID=89123977
diff --git a/src/test/java/com/google/devtools/build/lib/syntax/SkylarkEvaluationTest.java b/src/test/java/com/google/devtools/build/lib/syntax/SkylarkEvaluationTest.java
index 040b650..da38aa0 100644
--- a/src/test/java/com/google/devtools/build/lib/syntax/SkylarkEvaluationTest.java
+++ b/src/test/java/com/google/devtools/build/lib/syntax/SkylarkEvaluationTest.java
@@ -365,6 +365,20 @@
   }
 
   @Test
+  public void testForLoopMultipleVariables() throws Exception {
+    List<Statement> input = parseFileForSkylark(
+        "def foo():\n"
+        + "  s = ''\n"
+        + "  for [i, j] in [[1, 2], [3, 4]]:\n"
+        + "    s = s + str(i) + str(j) + '.'\n"
+        + "  return s\n"
+        + "s = foo()");
+
+    exec(input, env);
+    assertEquals("12.34.", env.lookup("s"));
+  }
+
+  @Test
   public void testNoneAssignment() throws Exception {
     List<Statement> input = parseFileForSkylark(
         "def foo(x=None):\n"