Changed behavior of compound assignment (+=)

FULL PRODUCT VERSION :
java version "1.7.0-ea"
Java(TM) SE Runtime Environment (build 1.7.0-ea-b144)
Java HotSpot(TM) Client VM (build 21.0-b14, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 6.1.7601]

A DESCRIPTION OF THE PROBLEM :
A following function cannot be compiled in java 1.6 or less,. but it can be compiled in java 1.7.

public static void main(String[] args) {
       Object x = "x";
       String y = "y";
       x += i;
}

I read http://jdk7.java.net/preview/ and I didn't find this changed behavior in new features.

REGRESSION. Last worked in version 6u26

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Just write this function
public static void main(String[] args) {
       Object x = "x";
       String y = "y";
       x += i;
}
in any class and try to compile it in different version of jdk.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
I was expecting that it will not be compiled.

Test.java:6: incompatible types
found : java.lang.Object
required: java.lang.String
                x += i;
                ^
1 error
ACTUAL -
It was compiled.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------

public class Test {
public static void main(String[] args) {
Object x = "A";
String i = "B";
x += i;
}
}

---------- END SOURCE ----------