Pull the node traversal tests into their own class. Create a common tree

creation method for both that uses string nodes so that we can run tree
traversals and assert orderings based on the nodes' string values.

This permits us to have a more complex and realistic tree and avoids the
duplicated boilerplate in creating the tree.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=256432611

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

@@ -38,6 +38,7 @@
 import static com.google.javascript.rhino.testing.NodeSubject.assertNode;
 import static java.lang.Boolean.FALSE;
 import static java.lang.Boolean.TRUE;
+import static org.mockito.Mockito.verify;
 
 import com.google.common.base.Joiner;
 import com.google.common.base.Predicates;
@@ -70,7 +71,7 @@
 import org.junit.runners.Parameterized;
 import org.junit.runners.Parameterized.Parameter;
 import org.junit.runners.Parameterized.Parameters;
-import org.mockito.InOrder;
+import org.mockito.ArgumentCaptor;
 import org.mockito.Mockito;
 
 /**
@@ -3541,44 +3542,70 @@ public void testIsBundledGoogModule_onlyIfInScript() {
       IR.script(exprResult); // Call this for its side effect of modifying `exprResult`.
       assertThat(NodeUtil.isBundledGoogModuleCall(callNode)).isTrue();
     }
+  }
+
+  @RunWith(JUnit4.class)
+  public static class NodeTraversalTests {
 
     @Test
     public void testVisitPreOrder() {
-      Node parent = new Node(Token.GENERIC_TYPE);
-      Node child1 = new Node(Token.GENERIC_TYPE);
-      Node child2 = new Node(Token.GENERIC_TYPE);
-      parent.addChildToBack(child1);
-      parent.addChildToBack(child2);
-
       NodeUtil.Visitor visitor = Mockito.mock(NodeUtil.Visitor.class);
-
-      NodeUtil.visitPreOrder(parent, visitor);
-
-      InOrder visitOrder = Mockito.inOrder(visitor);
-      visitOrder.verify(visitor).visit(parent);
-      visitOrder.verify(visitor).visit(child1);
-      visitOrder.verify(visitor).visit(child2);
+      NodeUtil.visitPreOrder(buildTestTree(), visitor);
+      assertThat(getNodeStringsFromMockVisitor(visitor))
+          .containsExactly("A", "B", "C", "D", "E", "F", "G")
+          .inOrder();
     }
 
     @Test
     public void testVisitPostOrder() {
-      Node parent = new Node(Token.GENERIC_TYPE);
-      Node child1 = new Node(Token.GENERIC_TYPE);
-      Node child2 = new Node(Token.GENERIC_TYPE);
-      parent.addChildToBack(child1);
-      parent.addChildToBack(child2);
-
       NodeUtil.Visitor visitor = Mockito.mock(NodeUtil.Visitor.class);
+      NodeUtil.visitPostOrder(buildTestTree(), visitor);
+      assertThat(getNodeStringsFromMockVisitor(visitor))
+          .containsExactly("C", "D", "B", "F", "G", "E", "A")
+          .inOrder();
+    }
 
-      NodeUtil.visitPostOrder(parent, visitor);
+    /**
+     * Builds a node tree of string {@link com.google.javascript.rhino.Node} with string labels that
+     * can be used to verify order in tests.
+     *
+     * <p>The resultant tree looks like this.
+     *
+     * <pre>
+     *      A
+     *    ↙   ↘
+     *   B     E
+     *  ↙ ↘    ↙ ↘
+     * C   D  F   G
+     * </pre>
+     */
+    private static Node buildTestTree() {
+      Node a = Node.newString("A");
+
+      Node b = Node.newString("B");
+      b.addChildToBack(Node.newString("C"));
+      b.addChildToBack(Node.newString("D"));
+      a.addChildToBack(b);
+
+      Node e = Node.newString("E");
+      e.addChildToBack(Node.newString("F"));
+      e.addChildToBack(Node.newString("G"));
+      a.addChildToBack(e);
 
-      InOrder visitOrder = Mockito.inOrder(visitor);
-      visitOrder.verify(visitor).visit(child1);
-      visitOrder.verify(visitor).visit(child2);
-      visitOrder.verify(visitor).visit(parent);
+      return a;
+    }
+
+    private static ImmutableList<String> getNodeStringsFromMockVisitor(
+        NodeUtil.Visitor mockVisitor) {
+      ArgumentCaptor<Node> captor = ArgumentCaptor.forClass(Node.class);
+      verify(mockVisitor, Mockito.atLeastOnce()).visit(captor.capture());
+      return captor.getAllValues().stream()
+          .map(n -> n.getString())
+          .collect(ImmutableList.toImmutableList());
     }
   }
 
+
   @RunWith(Parameterized.class)
   public static final class ReferencesThisTest {
     @Parameters(name = "{0} node in \"{1}\" referencesThis() should be {2}")