enhance unsafe side_effects (#4862)

closes #4861

lib/compress.js

@@ -1638,7 +1638,7 @@ merge(Compressor.prototype, {
         return node instanceof AST_SymbolRef && node.definition().undeclared;
     }
 
-    var global_names = makePredicate("Array Boolean clearInterval clearTimeout console Date decodeURI decodeURIComponent encodeURI encodeURIComponent Error escape eval EvalError Function isFinite isNaN JSON Math Number parseFloat parseInt RangeError ReferenceError RegExp Object setInterval setTimeout String SyntaxError TypeError unescape URIError");
+    var global_names = makePredicate("Array Boolean clearInterval clearTimeout console Date decodeURI decodeURIComponent encodeURI encodeURIComponent Error escape eval EvalError Function isFinite isNaN JSON Map Math Number parseFloat parseInt RangeError ReferenceError RegExp Object Set setInterval setTimeout String SyntaxError TypeError unescape URIError WeakMap WeakSet");
     AST_SymbolRef.DEFMETHOD("is_declared", function(compressor) {
         return this.defined
             || !this.definition().undeclared
@@ -4800,10 +4800,14 @@ merge(Compressor.prototype, {
     });
 
     var global_pure_fns = makePredicate("Boolean decodeURI decodeURIComponent Date encodeURI encodeURIComponent Error escape EvalError isFinite isNaN Number Object parseFloat parseInt RangeError ReferenceError String SyntaxError TypeError unescape URIError");
+    var global_pure_constructors = makePredicate("Map Set WeakMap WeakSet");
     AST_Call.DEFMETHOD("is_expr_pure", function(compressor) {
         if (compressor.option("unsafe")) {
             var expr = this.expression;
-            if (is_undeclared_ref(expr) && global_pure_fns[expr.name]) return true;
+            if (is_undeclared_ref(expr)) {
+                if (global_pure_fns[expr.name]) return true;
+                if (this instanceof AST_New && global_pure_constructors[expr.name]) return true;
+            }
             if (expr instanceof AST_Dot && is_undeclared_ref(expr.expression)) {
                 var static_fn = static_fns[expr.expression.name];
                 return static_fn && (static_fn[expr.property]

test/compress/side_effects.js

@@ -198,6 +198,36 @@ global_fns: {
     ]
 }
 
+global_constructors: {
+    options = {
+        side_effects: true,
+        unsafe: true,
+    }
+    input: {
+        Map;
+        new Map(console.log("foo"));
+        Set;
+        new Set(console.log("bar"));
+        WeakMap;
+        new WeakMap(console.log("baz"));
+        WeakSet;
+        new WeakSet(console.log("moo"));
+    }
+    expect: {
+        console.log("foo");
+        console.log("bar");
+        console.log("baz");
+        console.log("moo");
+    }
+    expect_stdout: [
+        "foo",
+        "bar",
+        "baz",
+        "moo",
+    ]
+    node_version: ">=0.12"
+}
+
 unsafe_builtin_1: {
     options = {
         side_effects: true,