Add mechanism for type coercion in JS

Use it for char boxing/unboxing and unit materialization.
Possible to use for other purposes, for example, to add type checks
to dynamics.

See KT-18793, KT-17915, KT-19081, KT-18216, KT-12970, KT-17014,
KT-13932, KT-13930

compiler/testData/codegen/box/casts/lambdaToUnitCast.kt

@@ -1,6 +1,3 @@
-// IGNORE_BACKEND: JS
-// JS backend does not support Unit well. See KT-13932
-
 val foo: () -> Unit = {}
 
 fun box(): String {

compiler/testData/codegen/box/casts/unitAsAny.kt

@@ -1,6 +1,3 @@
-// TODO: muted automatically, investigate should it be ran for JS or not
-// IGNORE_BACKEND: JS
-
 fun println(s: String) {
 }
 

compiler/testData/codegen/box/casts/unitAsSafeAny.kt

@@ -1,6 +1,3 @@
-// TODO: muted automatically, investigate should it be ran for JS or not
-// IGNORE_BACKEND: JS
-
 fun println(s: String) {
 }
 

compiler/testData/codegen/box/coroutines/unitTypeReturn/suspendNonLocalReturn.kt

@@ -1,6 +1,5 @@
 // WITH_RUNTIME
 // WITH_COROUTINES
-// IGNORE_BACKEND: JS
 import helpers.*
 import kotlin.coroutines.experimental.*
 import kotlin.coroutines.experimental.intrinsics.*

compiler/testData/codegen/box/primitiveTypes/equalityWithObject/generated/primitiveEqObjectChar.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JS
 // Auto-generated by GeneratePrimitiveVsObjectEqualityTestData. Do not edit!
 
 val nx: Any? = '0'

compiler/testData/codegen/box/properties/kt4383.kt

@@ -1,6 +1,3 @@
-// TODO: muted automatically, investigate should it be ran for JS or not
-// IGNORE_BACKEND: JS
-
 import kotlin.reflect.KProperty
 
 class D {

compiler/testData/codegen/box/unit/UnitValue.kt

@@ -1,6 +1,3 @@
-// TODO: muted automatically, investigate should it be ran for JS or not
-// IGNORE_BACKEND: JS
-
 fun foo() {}
 
 fun box(): String {

compiler/testData/codegen/box/unit/kt4212.kt

@@ -1,6 +1,3 @@
-// TODO: muted automatically, investigate should it be ran for JS or not
-// IGNORE_BACKEND: JS
-
 fun foo(): Any? = bar()
 
 fun bar() {}