Update the AstValidator to better validate destructuring assignments.

First, reject destructuring on the LHS of compound assignments.
Second, accept full LValue expressions as well as names in patterns in the LHS of DEFAULT_VALUE nodes when used on the LHS of a assignment expression.

For example this expression was rejected:
  ([ x['y'] = z ] = [])

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=171842831

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

@@ -240,6 +240,8 @@ public void validateExpression(Node n) {
 
       // Assignments
       case ASSIGN:
+        validateAssignmentExpression(n);
+        return;
       case ASSIGN_BITOR:
       case ASSIGN_BITXOR:
       case ASSIGN_BITAND:
@@ -252,7 +254,7 @@ public void validateExpression(Node n) {
       case ASSIGN_DIV:
       case ASSIGN_MOD:
       case ASSIGN_EXPONENT:
-        validateAssignmentExpression(n);
+        validateCompoundAssignmentExpression(n);
         return;
 
       case HOOK:
@@ -770,19 +772,11 @@ private void validateParameters(Node n) {
     }
   }
 
-  private void validateDefaultValue(Token type, Node n) {
+  private void validateDefaultValue(Token contextType, Node n) {
     validateFeature(Feature.DEFAULT_PARAMETERS, n);
-    validateAssignmentExpression(n);
-    Node lhs = n.getFirstChild();
-
-    // LHS can only be a name or destructuring pattern.
-    if (lhs.isName()) {
-      validateName(lhs);
-    } else if (lhs.isArrayPattern()) {
-      validateArrayPattern(type, lhs);
-    } else {
-      validateObjectPattern(type, lhs);
-    }
+    validateChildCount(n);
+    validateLHS(contextType, n.getFirstChild());
+    validateExpression(n.getLastChild());
   }
 
   private void validateCall(Node n) {
@@ -1171,6 +1165,24 @@ private void validateAssignmentExpression(Node n) {
     validateExpression(n.getLastChild());
   }
 
+  private void validateCompoundAssignmentExpression(Node n) {
+    validateChildCount(n);
+    Token contextType = n.getToken();
+    Node lhs = n.getFirstChild();
+    switch (lhs.getToken()) {
+      case NAME:
+        validateName(lhs);
+        break;
+      case GETPROP:
+      case GETELEM:
+        validateGetPropGetElemInLHS(contextType, lhs);
+        break;
+      default:
+        violation("Invalid child for " + contextType + " node", lhs);
+    }
+    validateExpression(n.getLastChild());
+  }
+
   private void validateGetProp(Node n) {
     validateNodeType(Token.GETPROP, n);
     validateChildCount(n);

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

@@ -164,12 +164,76 @@ public void testAwaitExpressionNoFunction() {
     expectInvalid(n, Check.EXPRESSION);
   }
 
-  public void testValidDestructuringAssignment() {
+  public void testInvalidArrayPattern0() {
     setAcceptedLanguage(LanguageMode.ECMASCRIPT_2015);
+
+    setAcceptedLanguage(LanguageMode.ECMASCRIPT_2015);
+
+    // [...x = 1] = [];
+    Node n = IR.assign(
+        new Node(Token.ARRAY_PATTERN,
+            new Node(Token.REST,
+                new Node(Token.DEFAULT_VALUE,
+                    IR.name("x"), IR.arraylit()))),
+        IR.arraylit());
+    expectInvalid(n, Check.EXPRESSION);
+  }
+
+  public void testValidDestructuringAssignment0() {
+    setAcceptedLanguage(LanguageMode.ECMASCRIPT_2015);
+    valid("var [x] = obj;");
+    valid("var [x = 1] = obj;");
+    valid("var [...y] = obj;");
     valid("var [x, ...y] = obj;");
+
+    valid("[x] = [];");
+    valid("[x = 1] = [];");
+    valid("[x.y] = [];");
+    valid("[x.y = 1] = [];");
+    valid("[x['y']] = [];");
+    valid("[x['y'] = 1] = [];");
+    valid("[x().y] = [];");
+    valid("[x().y = 1] = [];");
+    valid("[x()['y']] = [];");
+    valid("[x()['y'] = 1] = [];");
+
+    valid("([...y] = obj);");
+    valid("([x, ...y] = obj);");
     valid("([...this.x] = obj);");
+    valid("([...this['x']] = obj);");
+    valid("([...x.y] = obj);");
+    valid("([...x['y']] = obj);");
+    valid("([...x().y] = obj);");
+    valid("([...x()['y']] = obj);");
+  }
+
+  public void testValidDestructuringAssignment1() {
+    setAcceptedLanguage(LanguageMode.ECMASCRIPT_2015);
     valid("var {a:b} = obj;");
-    valid("({a:this.b} = obj);");
+    valid("({a:b} = obj);");
+    valid("({a:b.c} = obj);");
+    valid("({a:b().c} = obj);");
+    valid("({a:b['c']} = obj);");
+    valid("({a:b()['c']} = obj);");
+    valid("({a:b.c = 1} = obj);");
+    valid("({a:b().c = 1} = obj);");
+    valid("({a:b['c'] = 1} = obj);");
+    valid("({a:b()['c'] = 1} = obj);");
+  }
+
+  public void testValidDestructuringAssignment2() {
+    setAcceptedLanguage(LanguageMode.ECMASCRIPT_2015);
+    valid("var {['a']:b} = obj;");
+    valid("({['a']:b} = obj);");
+    valid("({['a']:this.b} = obj);");
+    valid("({['a']:b.c} = obj);");
+    valid("({['a']:b.c = 1} = obj);");
+    valid("({['a']:b().c} = obj);");
+    valid("({['a']:b().c = 1} = obj);");
+    valid("({['a']:b['c']} = obj);");
+    valid("({['a']:b['c'] = 1} = obj);");
+    valid("({['a']:b()['c']} = obj);");
+    valid("({['a']:b()['c'] = 1} = obj);");
   }
 
   public void testInvalidDestructuringAssignment() {