preserve constant modification under strict mode (#2717)

lib/compress.js

@@ -3279,7 +3279,12 @@ merge(Compressor.prototype, {
         });
         def(AST_Assign, function(compressor){
             var left = this.left;
-            if (left.has_side_effects(compressor)) return this;
+            if (left.has_side_effects(compressor)
+                || compressor.has_directive("use strict")
+                    && left instanceof AST_PropAccess
+                    && left.expression.is_constant()) {
+                return this;
+            }
             this.write_only = true;
             while (left instanceof AST_PropAccess) {
                 left = left.expression;

test/compress/pure_getters.js

@@ -348,6 +348,57 @@ set_immutable_4: {
     expect_stdout: true
 }
 
+set_immutable_5: {
+    options = {
+        collapse_vars: true,
+        conditionals: true,
+        evaluate: true,
+        pure_getters: "strict",
+        reduce_funcs: true,
+        reduce_vars: true,
+        side_effects: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        "use strict";
+        var a = 1;
+        a.foo += "";
+        if (a.foo) console.log("FAIL");
+        else console.log("PASS");
+    }
+    expect: {
+        "use strict";
+        1..foo += "";
+        1..foo ? console.log("FAIL") : console.log("PASS");
+    }
+    expect_stdout: true
+}
+
+set_immutable_6: {
+    options = {
+        collapse_vars: true,
+        conditionals: true,
+        evaluate: true,
+        pure_getters: "strict",
+        reduce_funcs: true,
+        reduce_vars: true,
+        side_effects: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a = 1;
+        a.foo += "";
+        if (a.foo) console.log("FAIL");
+        else console.log("PASS");
+    }
+    expect: {
+        1..foo ? console.log("FAIL") : console.log("PASS");
+    }
+    expect_stdout: true
+}
+
 set_mutable_1: {
     options = {
         collapse_vars: true,