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

js/js.ast/src/org/jetbrains/kotlin/js/backend/ast/metadata/metadataProperties.kt

@@ -24,6 +24,7 @@ import org.jetbrains.kotlin.descriptors.DeclarationDescriptor
 import org.jetbrains.kotlin.descriptors.FunctionDescriptor
 import org.jetbrains.kotlin.js.backend.ast.*
 import org.jetbrains.kotlin.resolve.inline.InlineStrategy
+import org.jetbrains.kotlin.types.KotlinType
 
 var JsName.staticRef: JsNode? by MetadataProperty(default = null)
 
@@ -56,12 +57,16 @@ var JsParameter.hasDefaultValue: Boolean by MetadataProperty(default = false)
 
 var JsInvocation.typeCheck: TypeCheck? by MetadataProperty(default = null)
 
-var JsInvocation.boxing: Boolean by MetadataProperty(default = false)
+var JsInvocation.boxing: BoxingKind by MetadataProperty(default = BoxingKind.NONE)
 
 var JsVars.exportedPackage: String? by MetadataProperty(default = null)
 
 var JsExpressionStatement.exportedTag: String? by MetadataProperty(default = null)
 
+var JsExpression.type: KotlinType? by MetadataProperty(default = null)
+
+var JsExpression.isUnit: Boolean by MetadataProperty(default = false)
+
 /**
  * For function and lambda bodies indicates what declaration corresponds to.
  * When absent (`null`) on body of a named function, this function is from external JS module.
@@ -142,4 +147,10 @@ enum class SpecialFunction(val suggestedName: String) {
     WRAP_FUNCTION("wrapFunction"),
     TO_BOXED_CHAR("toBoxedChar"),
     UNBOX_CHAR("unboxChar"),
+}
+
+enum class BoxingKind {
+    NONE,
+    BOXING,
+    UNBOXING
 }