Remove unnecessary fallthrough case labels

eg.
switch('foo') {
  case 'foo':
  case 'bar':
    alert();
}

becomes

alert();

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=163541301

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

@@ -374,7 +374,10 @@ private Node tryOptimizeSwitch(Node n) {
 
     // Generally, it is unsafe to remove other cases when the default case is not the last one.
     if (defaultCase == null || n.getLastChild().isDefaultCase()) {
-      Node cond = n.getFirstChild(), prev = null, next = null, cur;
+      Node cond = n.getFirstChild();
+      Node prev = null;
+      Node next = null;
+      Node cur;
 
       for (cur = cond.getNext(); cur != null; cur = next) {
         next = cur.getNext();
@@ -404,18 +407,33 @@ private Node tryOptimizeSwitch(Node n) {
             removeCase(n, cur);
           }
         }
-        if (caseMatches != TernaryValue.UNKNOWN) {
-          Node block, lastStm;
+        if (cur != null && caseMatches == TernaryValue.TRUE) {
           // Skip cases until you find one whose last stm is a removable break
+          Node matchingCase = cur;
+          Node matchingCaseBlock = matchingCase.getLastChild();
           while (cur != null) {
-            block = cur.getLastChild();
-            lastStm = block.getLastChild();
-            cur = cur.getNext();
+            Node block = cur.getLastChild();
+            Node lastStm = block.getLastChild();
+            boolean isLastStmRemovableBreak = false;
             if (lastStm != null && isExit(lastStm)) {
               removeIfUnnamedBreak(lastStm);
+              isLastStmRemovableBreak = true;
+            }
+            next = cur.getNext();
+            // Remove the fallthrough case labels
+            if (cur != matchingCase) {
+              while (block.hasChildren()) {
+                matchingCaseBlock.addChildToBack(block.getFirstChild().detach());
+              }
+              cur.detach();
+              reportCodeChange();
+            }
+            cur = next;
+            if (isLastStmRemovableBreak) {
               break;
             }
           }
+
           // Remove any remaining cases
           for (; cur != null; cur = next) {
             next = cur.getNext();

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

@@ -407,22 +407,39 @@ public void testOptimizeSwitch() {
         "  break;\n" +
         "}",
         "");
-    foldSame("switch ('fallThru') {\n" +
-        "case 'fallThru':\n" +
-        "  if (foo(123) > 0) {\n" +
-        "    foobar(1);\n" +
-        "    break;\n" +
-        "  }\n" +
-        "  foobar(2);\n" +
-        "case 'bar':\n" +
-        "  bar();\n" +
-        "}");
-    foldSame("switch ('fallThru') {\n" +
-        "case 'fallThru':\n" +
-        "  foo();\n" +
-        "case 'bar':\n" +
-        "  bar();\n" +
-        "}");
+    fold(
+        LINE_JOINER.join(
+            "switch ('fallThru') {",
+            "case 'fallThru':",
+            "  if (foo(123) > 0) {",
+            "    foobar(1);",
+            "    break;",
+            "  }",
+            "  foobar(2);",
+            "case 'bar':",
+            "  bar();",
+            "}"),
+        LINE_JOINER.join(
+            "switch ('fallThru') {",
+            "case 'fallThru':",
+            "  if (foo(123) > 0) {",
+            "    foobar(1);",
+            "    break;",
+            "  }",
+            "  foobar(2);",
+            "  bar();",
+            "}"));
+    fold(
+        LINE_JOINER.join(
+            "switch ('fallThru') {",
+            "case 'fallThru':",
+            "  foo();",
+            "case 'bar':",
+            "  bar();",
+            "}"),
+        LINE_JOINER.join(
+            "foo();",
+            "bar();"));
     fold(
         LINE_JOINER.join(
             "switch ('hasDefaultCase') {",
@@ -496,11 +513,14 @@ public void testOptimizeSwitch() {
         "case '\\u000B':\n" +
         "  foo();\n" +
         "}");
-    foldSame("switch ('empty') {\n" +
-        "case 'empty':\n" +
-        "case 'foo':\n" +
-        "  foo();\n" +
-        "}");
+    fold(
+        LINE_JOINER.join(
+            "switch ('empty') {",
+            "case 'empty':",
+            "case 'foo':",
+            "  foo();",
+            "}"),
+        "foo()");
 
     fold(
         LINE_JOINER.join(
@@ -547,6 +567,25 @@ public void testOptimizeSwitch2() {
         "outer: {f(); break outer;}");
   }
 
+  public void testOptimizeSwitch3() {
+    fold(
+        LINE_JOINER.join(
+            "switch (1) {",
+            "  case 1:",
+            "  case 2:",
+            "  case 3: {",
+            "    break;",
+            "  }",
+            "  case 4:",
+            "  case 5:",
+            "  case 6:",
+            "  default:",
+            "    fail('Should not get here');",
+            "    break;",
+            "}"),
+            "");
+  }
+
   public void testOptimizeSwitchWithLabellessBreak() {
     test(
         LINE_JOINER.join(