Add few missing methods needed by "Tree" view in inspector

Commit 5000538 improved "Tree" view on Sprite AST but few methods
were missing in some classes causing inspector to crash on DNU
in some cases. This commit add these missing methods.

src/SpriteLang/ECon.class.st

@@ -17,9 +17,19 @@ ECon >> evaluateIn: aBindEnv [
 	^prim evaluateIn: aBindEnv 
 ]
 
+{ #category : #GT }
+ECon >> gtBind [
+	^''
+]
+
 { #category : #GT }
 ECon >> gtChildren [
-	^{ prim }
+	^#()
+]
+
+{ #category : #GT }
+ECon >> gtExpr [
+	^self
 ]
 
 { #category : #substitution }

src/SpriteLang/SpriteImm.class.st

@@ -19,6 +19,11 @@ immExpr :: Imm b -> F.Expr
 	self subclassResponsibility
 ]
 
+{ #category : #testing }
+SpriteImm >> isLet [
+	^false
+]
+
 { #category : #printing }
 SpriteImm >> printOn: aStream [
 	self printStructOn: aStream