Require superclass expressions to be lvals

And remove a test that violates this.

Closes #185

acorn.js

@@ -2406,6 +2406,7 @@
     next();
     node.id = tokType === _name ? parseIdent() : isStatement ? unexpected() : null;
     node.superClass = eat(_extends) ? parseExpression() : null;
+    if (node.superClass) checkLVal(node.superClass);
     var classBody = startNode();
     classBody.body = [];
     expect(_braceL);

test/tests-harmony.js

@@ -6465,72 +6465,6 @@ test("var A = class extends B {}", {
   locations: true
 });
 
-test("class A extends class B extends C {} {}", {
-  type: "Program",
-  body: [{
-    type: "ClassDeclaration",
-    id: {
-      type: "Identifier",
-      name: "A",
-      loc: {
-        start: {line: 1, column: 6},
-        end: {line: 1, column: 7}
-      }
-    },
-    superClass: {
-      type: "ClassExpression",
-      id: {
-        type: "Identifier",
-        name: "B",
-        loc: {
-          start: {line: 1, column: 22},
-          end: {line: 1, column: 23}
-        }
-      },
-      superClass: {
-        type: "Identifier",
-        name: "C",
-        loc: {
-          start: {line: 1, column: 32},
-          end: {line: 1, column: 33}
-        }
-      },
-      body: {
-        type: "ClassBody",
-        body: [],
-        loc: {
-          start: {line: 1, column: 34},
-          end: {line: 1, column: 36}
-        }
-      },
-      loc: {
-        start: {line: 1, column: 16},
-        end: {line: 1, column: 36}
-      }
-    },
-    body: {
-      type: "ClassBody",
-      body: [],
-      loc: {
-        start: {line: 1, column: 37},
-        end: {line: 1, column: 39}
-      }
-    },
-    loc: {
-      start: {line: 1, column: 0},
-      end: {line: 1, column: 39}
-    }
-  }],
-  loc: {
-    start: {line: 1, column: 0},
-    end: {line: 1, column: 39}
-  }
-}, {
-  ecmaVersion: 6,
-  ranges: true,
-  locations: true
-});
-
 test("class A {get() {}}", {
   type: "Program",
   body: [{