A few basic tests.

test/compress/strict-mode.js

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+keep_non_reserved_in_non_strict_mode: {
+    input: {
+        function outer() {
+            var implements = 1;
+            var interface = 1;
+            var let = 1;
+            var package = 1;
+            var private = 1;
+            var protected = 1;
+            var public = 1;
+            var static = 1;
+
+            function inner() {
+                implements++;
+                interface++;
+                let++;
+                package++;
+                private++;
+                protected++;
+                public++;
+                static++;
+            }
+        }
+    }
+    expect: {
+        function outer() {
+            var implements = 1;
+            var interface = 1;
+            var let = 1;
+            var package = 1;
+            var private = 1;
+            var protected = 1;
+            var public = 1;
+            var static = 1;
+
+            function inner() {
+                implements++;
+                interface++;
+                let++;
+                package++;
+                private++;
+                protected++;
+                public++;
+                static++;
+            }
+        }
+    }
+}
+
+mangle_non_reserved_in_non_strict_mode: {
+    mangle = {}
+    input: {
+        function outer() {
+            var implements = 1;
+            var interface = 1;
+            var let = 1;
+            var package = 1;
+            var private = 1;
+            var protected = 1;
+            var public = 1;
+            var static = 1;
+
+            function inner() {
+                implements++;
+                interface++;
+                let++;
+                package++;
+                private++;
+                protected++;
+                public++;
+                static++;
+            }
+        }
+    }
+    expect: {
+        function outer() {
+            var r=1;
+            var a=1;
+            var v=1;
+            var n=1;
+            var o=1;
+            var t=1;
+            var u=1;
+            var c=1;
+            function f() {
+                r++;
+                a++;
+                v++;
+                n++;
+                o++;
+                t++;
+                u++;
+                c++
+            }
+        }
+    }
+}
+

test/strict-mode-mangler.js

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+var UglifyJS = require("..");
+var assert = require("assert");
+
+var alphabet = "abcdefghijklmnop".split("");
+var perms = [];
+var linesOfCode = [];
+var startTime = new Date().getTime();
+
+console.log("--- Strict mode mangling tests");
+
+function permute(text) {
+    perms.push(text);
+    if(text.length < 4) { 
+        for(var i = 0; i < alphabet.length; i++) {
+            permute(text + alphabet[i]);
+        }
+    }
+}
+permute("");
+
+var permuteTime = new Date().getTime() - startTime;
+
+console.log("  - Generated perms in " + permuteTime + "ms")
+console.log("  - Testing with " + perms.length + " identifiers");
+console.log("  - Mangling (this will take some time)...");
+
+function getCode(withStrictMode) {
+
+    linesOfCode = "        function outer() {\n";
+    if (!!withStrictMode) {
+        linesOfCode += "        \"use strict\";\n";
+    }
+    for (var i in perms) {
+        linesOfCode += "            var _" + perms[i] + " = 1;\n"
+    }
+    linesOfCode += "            function inner() {\n";       
+    for (var i in perms) {
+        linesOfCode += "                 _" + perms[i] + "++;\n";
+    }
+    linesOfCode += "            }\n";
+    linesOfCode += "        }\n";
+
+    return linesOfCode;
+
+}
+
+module.exports = function () {
+    
+    var startManglingTime = new Date().getTime();
+    var mangled_code = mangle(getCode(false));
+    
+    console.log("  - Found 'var let' at char: " + mangled_code.indexOf('var let'));
+    console.log("  - Mangled code with no strict mode in " + Math.floor((new Date().getTime() - startManglingTime)/ 1000) + "s");
+    assert(mangled_code.indexOf('var let') >= 0, "No 'var let' produced in the mangled code outside strict mode.");  
+
+    startManglingTime = new Date().getTime();
+    var mangled_strict_code = mangle(getCode(true));   
+    console.log("  - This should be -1: " + mangled_strict_code.indexOf('var let'));
+    console.log("  - Mangled code with strict mode in " + Math.floor((new Date().getTime() - startManglingTime)/ 1000) + "s");
+    assert(mangled_strict_code.indexOf('var let') === -1, "A 'var let' statement was produced inside strict mode.");
+    
+}
+
+function mangle(js) {
+    var stream = UglifyJS.OutputStream();
+    var parsed = UglifyJS.parse(js);
+    parsed.figure_out_scope();
+    parsed.mangle_names();
+    parsed.print(stream);
+    return stream.toString();
+}
+
+// Run standalone
+if (module.parent === null) {
+    module.exports();
+}
+