Make class property assignment pure.

Closes #724

lib/compress/index.js

@@ -2466,6 +2466,10 @@ function is_undefined(node, compressor) {
             if (this.properties[i]._dot_throw(compressor)) return true;
         return false;
     });
+    // Do not be as strict with classes as we are with objects.
+    // Hopefully the community is not going to abuse static getters and setters.
+    // https://github.com/terser/terser/issues/724#issuecomment-643655656
+    def_may_throw_on_access(AST_Class, return_false);
     def_may_throw_on_access(AST_ObjectProperty, return_false);
     def_may_throw_on_access(AST_ObjectGetter, return_true);
     def_may_throw_on_access(AST_Expansion, function(compressor) {

test/compress/classes.js

@@ -79,3 +79,19 @@ class_duplication_2: {
     }
 }
 
+pure_prop_assignment_for_classes: {
+    options = {
+        defaults: true,
+        toplevel: true
+    }
+    input: {
+        class A {}
+        A.staticProp = "A"
+
+        class B {
+            static get danger() { }
+        }
+        B.staticProp = ""
+    }
+    expect: { }
+}