Update String#contentEquals(java.lang.CharSequence)

Currently String#contentEquals(java.lang.CharSequence) implementation
only handles if the bytes in the String value are UTF16 encoded. If not
it throws an assertion error like the following:

    [junit] java.lang.AssertionError: Currently only UTF16 is supported for String comparision with an instance of type AbstractStringBuilder
    [junit]     at java.lang.String.contentEquals(String.java:275)
    [junit]     at gov.nasa.jpf.test.java.lang.StringTest.testContentEquals(StringTest.java:301)
    [junit]     at java.lang.reflect.Method.invoke(gov.nasa.jpf.vm.JPF_java_lang_reflect_Method)
    [junit]     at gov.nasa.jpf.util.test.TestJPF.runTestMethod(TestJPF.java:648)

This updates String#contentEquals(java.lang.CharSequence) method, so it
handles both LATIN1 and UTF16 encoded Strings.

src/classes/modules/java.base/java/lang/String.java

@@ -253,6 +253,26 @@ public boolean contentEquals(StringBuffer stringBuffer){
 		}
 	}
 
+	private boolean nonSyncContentEquals(AbstractStringBuilder abstractStringBuilder) {
+		int len = length();
+		if (len != abstractStringBuilder.length()) {
+			return false;
+		}
+		byte v1[] = value;
+		byte v2[] = abstractStringBuilder.getValue();
+		if (coder() == abstractStringBuilder.getCoder()) {
+			int n = v1.length;
+			for (int i = 0; i < n; i++) {
+				if (v1[i] != v2[i]) {
+					return false;
+				}
+			}
+		} else {
+			return isLatin1() && StringUTF16.contentEquals(v1, v2, len);
+		}
+		return true;
+	}
+
   native static boolean equals0 (char[] a, char[] b, int len);
 
   /**
@@ -264,21 +284,28 @@ public boolean contentEquals (CharSequence charSequence){
 			return false;
 		}
 
-		// that should be the common case (String)
-		if (charSequence.equals(this)){
-			return true;
+		// cs is a String
+		if (charSequence instanceof String) {
+			return equals(charSequence);
 		}
 
-		// we can do that natively, too
+		// cs is a StringBuffer, or StringBuilder
 		if (charSequence instanceof AbstractStringBuilder) {
-			byte[] val = ((AbstractStringBuilder) charSequence).getValue();
-			byte coder = ((AbstractStringBuilder) charSequence).getCoder();
-			assert coder == UTF16 : "Currently only UTF16 is supported for String comparision with an instance of type AbstractStringBuilder";
-			return StringUTF16.contentEquals(this.value, val, this.length());
+			if (charSequence instanceof StringBuffer) {
+				synchronized (charSequence) {
+					return nonSyncContentEquals((AbstractStringBuilder) charSequence);
+				}
+			} else {
+				return nonSyncContentEquals((AbstractStringBuilder) charSequence);
+			}
 		}
 
 		// generic CharSequence - expensive
 		int n = charSequence.length();
+		if (n != length()) {
+			return false;
+		}
+
 		byte[] val = this.value;
 		if (isLatin1()) {
 			for (int i = 0; i < n; i++) {