Assume properties may be read on a yield in DeadPropertyAssignmentsEl…

…imination

DeadPropertyAssignments is a function-local optimization, so it doesn't try to guess whether a property is read outside a function or not.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=191676794

src/com/google/javascript/jscomp/DeadPropertyAssignmentElimination.java

@@ -391,6 +391,7 @@ private boolean visitNode(Node n, Node parent) {
           }
           return true;
         case CALL:
+        case YIELD:
           if (ASSUME_CONSTRUCTORS_HAVENT_ESCAPED && isConstructor && !NodeUtil.referencesThis(n)
               && NodeUtil.getEnclosingType(n, Token.TRY) == null) {
             // this.x properties are okay.

test/com/google/javascript/jscomp/DeadPropertyAssignmentEliminationTest.java

@@ -194,6 +194,54 @@ public void testCall() {
             "  doSomething(this.c);",
             "  this.c = 30;",
             "}"));
+
+    // TODO(b/77600088): a.b.c = 20 is dead because of a.b.c = 25.
+    testSame(
+        lines(
+            "var foo = function() {",
+            "  a.b.c = 20;",
+            "  doSomething(a.b.c = 25);", // this should actually deaden "a.b.c = 20". oh well
+            "  a.b.c = 30;",
+            "}"));
+  }
+
+  public void testYield() {
+    // Assume that properties may be read during a yield
+    testSame(
+        lines(
+            "var foo = function*() {",
+            "  a.b.c = 20;",
+            "  yield;",
+            "  a.b.c = 30;",
+            "}"));
+
+    testSame(
+        lines(
+            "/** @constructor */",
+            "var foo = function*() {",
+            "  this.c = 20;",
+            "  yield;",
+            "  this.c = 30;",
+            "}"));
+
+    testSame(
+        lines(
+            "var obj = {",
+            "  *gen() {",
+            "    this.c = 20;",
+            "    yield;",
+            "    this.c = 30;",
+            "  }",
+            "}"));
+
+    // TODO(b/77600088): a.b.c = 20 is dead because of a.b.c = 25, and should be eliminated.
+    testSame(
+        lines(
+            "var foo = function*() {",
+            "  a.b.c = 20;",
+            "  yield a.b.c = 25;",
+            "  a.b.c = 30;",
+            "}"));
   }
 
   public void testUnknownLookup() {