Consider a ++ or -- expression as a read, if the result is used.

Removes some false positives for the "unused local variable" warning.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=106978097

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

@@ -530,8 +530,8 @@ boolean isAssignedOnceInLifetime() {
     }
 
     /**
-     * @return The one and only assignment. Returns if there are 0 or 2+
-     *    assignments.
+     * @return The one and only assignment. Returns null if the number of assignments is not
+     *     exactly one.
      */
     private Reference getOneAndOnlyAssignment() {
       Reference assignment = null;
@@ -715,9 +715,7 @@ boolean isInitializingDeclaration() {
     * return the assigned value, otherwise null.
     */
     Node getAssignedValue() {
-      Node parent = getParent();
-      return (parent.isFunction())
-          ? parent : NodeUtil.getAssignedValue(nameNode);
+      return NodeUtil.getRValueOfLValue(nameNode);
     }
 
     BasicBlock getBasicBlock() {
@@ -747,6 +745,11 @@ boolean isSimpleAssignmentToName() {
           && parent.getFirstChild() == nameNode;
     }
 
+    /**
+     * Returns whether the name node for this reference is an lvalue.
+     * TODO(tbreisacher): This method disagrees with NodeUtil#isLValue for
+     * "var x;" and "let x;". Consider updating it to match.
+     */
     boolean isLvalue() {
       Node parent = getParent();
       int parentType = parent.getType();

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

@@ -276,6 +276,10 @@ private void checkVar(Var v, List<Reference> references) {
               && !lhsOfForInLoop) {
             unusedAssignment = reference;
           }
+          if ((reference.getParent().isDec() || reference.getParent().isInc())
+              && NodeUtil.isExpressionResultUsed(reference.getNode())) {
+            isRead = true;
+          }
         } else {
           isRead = true;
         }

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

@@ -204,7 +204,7 @@ public void testUnusedLocalLet() {
     assertUnusedEs6("function f() { let a; a = 2; }");
   }
 
-  public void xtestUnusedLocalConst() {
+  public void testUnusedLocalConst() {
     enableUnusedLocalAssignmentCheck = true;
     assertUnusedEs6("function f() { const a = 2; }");
   }
@@ -224,6 +224,14 @@ public void testUnusedAssignedInInnerFunction() {
     assertUnused("function f() { var x = 1; function g() { x = 2; } }");
   }
 
+  public void testIncrementDecrementResultUsed() {
+    enableUnusedLocalAssignmentCheck = true;
+    assertNoWarning("function f() { var x = 5; while (x-- > 0) {} }");
+    assertNoWarning("function f() { var x = -5; while (x++ < 0) {} }");
+    assertNoWarning("function f() { var x = 5; while (--x > 0) {} }");
+    assertNoWarning("function f() { var x = -5; while (++x < 0) {} }");
+  }
+
   public void testUsedInInnerFunction() {
     enableUnusedLocalAssignmentCheck = true;
     assertNoWarning("function f() { var x = 1; function g() { use(x); } }");