Make CharSequence.length a function instead of property

And String.length as well.

This is done for JVM interoperability: java.lang.CharSequence is an open class
and has a function 'length()' which should be implemented in subclasses
somehow.

A minor unexpected effect of this is that String.length() is now a compile-time
constant (it wasn't such as a property because properties are not supported in
compile-time constant evaluation)

 #KT-3571 Fixed

js/js.libraries/src/core/string.kt

@@ -38,9 +38,6 @@ native public fun String.trim() : String = noImpl
 native("length")
 public val CharSequence.size: Int get() = noImpl
 
-library
-public fun CharSequence.length(): Int = noImpl
-
 library
 public fun CharSequence.isEmpty(): Boolean = noImpl
 

js/js.translator/src/org/jetbrains/k2js/translate/intrinsic/functions/factories/StringOperationFIF.java

@@ -27,7 +27,7 @@ public final class StringOperationFIF extends CompositeFIF {
 
     private StringOperationFIF() {
         add(pattern("kotlin", "CharSequence", "get").checkOverridden(), new BuiltInFunctionIntrinsic("charAt"));
-        add(pattern("kotlin.js", "length").isExtensionOf("kotlin.CharSequence"), LENGTH_PROPERTY_INTRINSIC);
+        add(pattern("kotlin", "CharSequence", "length").checkOverridden(), LENGTH_PROPERTY_INTRINSIC);
         add(pattern("kotlin.js", "isEmpty").isExtensionOf("kotlin.CharSequence"), IS_EMPTY_INTRINSIC);
     }
 }