Try the following steps,

Step 1:

npm update -g @angular/cli

Step 2:

Edit your package.json changing the line

"@angular/cli": "1.6.0",

to

"@angular/cli": "^1.6.0",

STEP 3:

npm update

Refer the steps

Step1: Edit your package.json changing the line

@angular/cli": "1.6.4"

to

@angular/cli": "^1.6.4"

Step2:

npm update -g @angular/cli 

Step3:

npm install --save-dev @angular/cli@latest

I was facing the exact same problem. These three did the tasks for me. You may like to see https://github.com/angular/angular-cli/issues/9307

try this out

npm install @angular-devkit/core --save-dev

https://github.com/angular/devkit/issues/256

try to update your angular cli

https://github.com/angular/angular-cli/issues/9307