JS: Make StringBuilder implement CharSequence and introduce secondary constructors.

libraries/stdlib/test/text/StringBuilderTest.kt

@@ -25,4 +25,41 @@ class StringBuilderTest {
             append("element", 2, 4)
         })
     }
+
+    @test fun asCharSequence() {
+        val original = "Some test string"
+        val sb = StringBuilder(original)
+        val result = sb.toString()
+        val cs = sb as CharSequence
+
+        assertEquals(result.length(), cs.length())
+        assertEquals(result.length(), sb.length())
+        for (index in result.indices) {
+            assertEquals(result[index], sb[index])
+            assertEquals(result[index], cs[index])
+        }
+        assertEquals(result.substring(2, 6), cs.subSequence(2, 6).toString())
+    }
+
+    @test fun constructors() {
+        StringBuilder().let { sb ->
+            assertEquals(0, sb.length())
+            assertEquals("", sb.toString())
+        }
+
+        StringBuilder(16).let { sb ->
+            assertEquals(0, sb.length())
+            assertEquals("", sb.toString())
+        }
+
+        StringBuilder("content").let { sb ->
+            assertEquals(7, sb.length())
+            assertEquals("content", sb.toString())
+        }
+
+        StringBuilder(StringBuilder("content")).let { sb ->
+            assertEquals(7, sb.length())
+            assertEquals("content", sb.toString())
+        }
+    }
 }